=head1 NAME

DBIx::DataModel::Doc::Reference - General reference for DBIx::DataModel

=head1 DOCUMENTATION CONTEXT

This chapter is part of the C<DBIx::DataModel> manual. 

=over

=item * 

L<SYNOPSIS AND DESCRIPTION|DBIx::DataModel>

=item * 

L<DESIGN|DBIx::DataModel::Doc::Design>

=item * 

L<QUICKSTART|DBIx::DataModel::Doc::Quickstart>

=item *

REFERENCE

=item *

L<MISC|DBIx::DataModel::Doc::Misc>

=item *

L<INTERNALS|DBIx::DataModel::Doc::Internals>

=item *

L<GLOSSARY|DBIx::DataModel::Doc::Glossary>

=back


B<WARNING> : Since version 2.0, a number of important changes
have been made in the API
(classes renamed, arguments renamed or reorganized, etc. -- see
L<DBIx::DataModel::Doc::Delta_v2>). 
If you need backwards compatibility, make sure to load the 
L<1.0 compatibility layer|DBIx::DataModel::Compatibility::V1>.



=head1 INTRODUCTION

This chapter is the detailed reference description
for most public methods in C<DBIx::DataModel>.

Just a couple of peripheric classes are described in 
separate documents : this is the case for 
L<DBIx::DataModel::Schema::Generator> (automatic schema generation
from external sources), and for 
L<DBIx::DataModel::Statement::JDBC> (special methods for working
with L<DBD::JDBC>).


=head1 META-SCHEMA DEFINITION

Most methods listed below for defining schema, tables, associations, etc.
come in two flavours :

=over

=item *

a "front-end" method, starting with an uppercase letter,
that uses positional parameters. This version is prefered
for conciseness and for backwards compatibility.

=item *

a "back-end" method, with a name of shape C<define_*>, 
that uses named parameters. This version is prefered for
completeness.

=back

The invocant for front-end methods can be either
the Schema class name, or the meta-schema instance.
The invocant for back-end methods must be 
the meta-schema instance.

=head2 Schema() 

  my $schema_class = DBIx::DataModel->Schema($schema_name, %options);

Front-end method for L</define_schema()>. The call above is equivalent to 

  my $meta_schema = DBIx::DataModel->define_schema(
    class => $schema_name,
    %options,
   );
  my $schema_class = $meta_schema->class;


=head2 define_schema()

  my $meta_schema = DBIx::DataModel->define_schema(
     class => $schema_name, 
     %options,
   );

Creates a new Perl class of name C<$schema_name> that represents a
database schema. That class inherits from L<DBIx::DataModel::Schema>.
At the same time, an instance of L<DBIx::DataModel::Meta::Schema> is
also created, for holding meta-information about that schema (for
example lists of classes, associations, types, etc. that will be
declared within the schema). Possible C<%options> are :

=over

=item class

Name of the Perl schema class to be created.

=item isa

Parent class(es) for that schema class (scalar or arrayref).
The default parent is L<DBIx::DataModel::Schema>.

=item auto_insert_columns

A hashref specifying columns to be automatically inserted
in every table. Keys of the hashref are column names, values
are handlers (coderefs). This can also be done separately for each
table (see the C<auto_insert_columns> parameter to 
L</define_table()>).

For example, each record could remember who created it and when, with
something like

  my $who_and_when = sub { $ENV{REMOTE_USER} . " at " . localtime };

  --->define_schema( 
        ...,
        auto_insert_colums => {created_by => $who_and_when},
      );


The handler code will be called as 

  $handler->(\%record, $table_class)

so that it can know something about its calling context.  In most
cases, however, the handler will not need these parameters, because it just
returns global information such as current user or current date/time.


=item auto_update_columns

A hashref specifying columns to be automatically updated in every
table. Keys of the hashref are column names, values are handlers
(coderefs). This specification can also be done separately for each
table (see the C<auto_update_columns> parameter to
L</define_table()>).  Such handlers are called at each update B<and>
each insert; if you use both C<auto_insert_columns> and
C<auto_update_columns>, make sure that the column names are not the
same.

For example, elaborating on the previous section, each record could
also remember the last modification with something like

  ...->define_schema( 
         ...,
         auto_insert_colums => {created_by => $who_and_when},
         auto_update_colums => {updated_by => $who_and_when},
       );


=item no_update_columns

A hashref specifying columns to be automatically ignored in
insert/update operations, for every table.  This can also be done
separately for each table (see the C<no_update_columns> parameter to
L</define_table()>). Keys of the hashref are column names, values are
indifferent (can be any true value).

This is useful for example when
some column are set up automatically by the database 
(like automatic time stamps or user identification).
It can also be useful if you want to temporarily add information
to memory objects, without passing it back to the database.

=item sql_no_inner_after_left_join

An optional boolean; if true, a LEFT OUTER JOIN in a multi-steps join
specification will force all subsequent joins to be also LEFT.
For example in the fake datamodel used in the rest of this documentation,
a join like

  HR->join(qw/Employee activities department/)->...

would be interpreted as 

  T_Employee LEFT OUTER JOIN T_Activity ON ...
             LEFT OUTER JOIN T_Department ON ...

even if the association betwen C<Activity> and C<Department> is 
many-to-one (which theoretically would result in a INNER JOIN by
default).

=item table_parent, join_parent

Optional application-specific classes, to be used instead of the
builtin classes, as parents for tables and joins declared in
this schema.

=item table_metaclass, join_metaclass, association_metaclass, path_metaclass, type_metaclass

Optional application-specific classes, to be used instead of the
builtin metaclasses, for instanciating meta-objects declared in this schema, .

=item statement_class

Optional application-specific class to be used instead of the
builtin class L<DBIx::DataModel::Statement> for instanciating 
statements.

=back



=head1 DATA DEFINITION (TABLES, ASSOCIATIONS, etc.)

This chapter deals with all methods for populating the
I<structure of a meta-schema> : defining tables, associations, types,
navigation methods, etc. Such objects are defined statically and will
not change during program execution.

By contrast, the L</"SCHEMAS">
chapter below deals with properties specific to each schema instance,
such as database connection, debugging properties, etc.

Reflection methods for navigating in the meta-schema structure and 
getting back information about tables, associations, etc.
are described in chapter L</"META-SCHEMA NAVIGATION">, at the
end of this document; such methods are seldom needed for regular
applications, but they can be useful for building tools around
C<DBIx::DataModel>,  such as CRUD frameworks, editors, translators, etc.


=head2 Meta-schema front-end methods

=head3 Table()

  $schema_class->Table($class_name, $db_name, @primary_key, \%options);

See L</define_table()>. The call above is equivalent to 

  $meta_schema->define_table(class       => $class_name,
                             db_name     => $db_name,
                             primary_key => \@primary_key,
                             %options);

=head3 View()

  $schema->View($class_name, $default_columns, $sql, 
                     \%where, @parent_tables);

This is another front-end method for 
L</"define_table()">,
for creating a new Perl class of name C<$class_name> that represents a
SQL SELECT request of shape 

  SELECT $columns FROM $db_tables [ WHERE %where ]

The call above is equivalent to 

  my @parents = map {$meta_schema->table($_)} @parent_tables;
  $schema->metadm->define_table(class           => $class_name,
                                db_name         => $sql,
                                where           => \%where,
                                default_columns => $default_columns,
                                parents         => \@parents);

The C<< @parent_tables >> list contains names of Perl 
table classes from which the view will also inherit.
If the SQL code in C<$sql> is a join between
several tables, then it is a good idea to mention these 
tables in C<< @parent_tables >>, so that their
path methods become available to instances
of this view. Be careful about table names : 
the SQL code in C<$sql> should contain I<database> table names,
whereas the members of C<< @parent_tables >> should be 
I<Perl> table classes.

Perl views as defined here have nothing to do with views declared in
the database itself. Perl views are totally unknown to the database,
they are just abstractions of SQL statements.  If you need to access
I<database views>, just use the C<Table()> declaration, like for a regular
table.

=head3 Association() 

  $schema->Association([$class1, $role1, $multiplicity1, @columns1],
                       [$class2, $role2, $multiplicity2, @columns2]);


See L</define_association()>. The call above is equivalent to 

  $schema->metadm->define_association(
    A => {table        => $class1->metadm, role       => $role1, 
          multiplicity => $multiplicity1,  join_cols  => \@columns1},
    B => {table        => $class2->metadm, role       => $role2, 
          multiplicity => $multiplicity2,  join_cols  => \@columns2},
    kind => 'Association',
  );

=head3 Composition() 

  $schema->Composition([$class1, $role1, $multiplicity1, @columns1],
                       [$class2, $role2, $multiplicity2, @columns2]);


See L</define_association()>. The call above is equivalent to 

  $schema->metadm->define_association(
    A => {table        => $class1->metadm, role       => $role1, 
          multiplicity => $multiplicity1,  join_cols  => \@columns1},
    B => {table        => $class2->metadm, role       => $role2, 
          multiplicity => $multiplicity2,  join_cols  => \@columns2},
    kind => 'Composition',
  );

See also L</"Special semantics for compositions"> below.

=head3 Type()

  $schema->Type($type_name => %handlers);

See L</define_type()>. The call above is equivalent to 

  $schema->metadm->define_type(name => $type_name, handlers => \%handlers);


=head2 Meta-schema back-end methods

=head3 define_table()

  $meta_schema->define_table(%options);

Creates a new Perl class of name C<$class_name> that represents a
database table. The new class inherits from the C<table_parent> 
declared in the schema (by default : L<DBIx::DataModel::Source::Table>).
At the same time, an instance of the C<table_metaclass> is also
created (by default : L<DBIx::DataModel::Meta::Source::Table>),
for holding meta-information about that table (database name, 
primary key, paths to other tables, etc.).

Returns C<$meta_schema>. Entries in C<%options> are :

=over

=item class => $string

Name of the class to be created.  If C<< $class_name >> contains no
C<::>, then the schema name is prepended to it (so the new table class
is created in the namespace of the schema, which is a recommended best
practice).

=item db_name => $string

SQL name of the database table or view that will be accessed through
this class.  Actually, it might be any SQL clause, like for example
C<Table1 t1 INNER JOIN Table2 t2 ON t1.fk=t2.pk>; see the L</View()>
method below.

=item primary_key => $string | \@array

Name of the column (or list of columns) that hold the primary key for
that table.

=item default_columns => $string

Whatever will be injected into SELECT statements, when no
C<-columns> parameter is explicitly supplied. The default
is C<'*'>.

=item where => \%hash | \@array

An optional reference to a WHERE condition, in 
L<SQL::Abstract::More> format. That condition
will be automatically injected into every SELECT statement
on this table. When working with regular tables, this parameter
is always empty; it is only useful for declaring C<DBIx::DataModel> 
views (see the L</View()> method below).

=item parents => [$parent1, ...]

List of meta-tables from which this table will inherit.  This can be
useful if your database supports table inheritance (like for example
PostgreSQL), and you want to reflect the inheritance structure 
within the Perl table classes.


=item column_types

An optional hashref, where keys are type names and values are arrayrefs
of columns on which this type should be applied; see L</define_column_type()>.

=item column_handlers

An optional hashref, where keys are column names and values are 
hashrefs of C<< { $handler_name => $handler_body } >>;
see L</define_column_handlers()>.

Then, when you call C<< $obj->apply_column_handler($handler_name) >>,
each column having a handler of the corresponding name will execute the
associated code. 

=item auto_insert_columns

A hashref of column names and handlers, 
like in L</define_schema()>, but only for one specific table.



=item auto_update_columns

A hashref of column names and handlers, 
like in L</define_schema()>, but only for one specific table.

=item no_update_columns

A hashref of column names, 
like in L</define_schema()>, but only for one specific table.

=back


=head3 define_association()

=head4 Features common to all kinds of associations

  $meta_schema->define_association(
    name   => $association_name,          # optional
    kind   => $kind,                      # 'Association' or 'Composition'
    A      => {
      table        => $meta_table_instance,
      role         => $role_name,         # optional
      multiplicity => $multiplicity_spec, # ex. "1..*"
      join_cols    => [$col1, ...]        # optional
    },
    B      => { ... },                    # same structure as 'A'
  );


Declares an association between two tables (or even two instances of
the same table). The arguments are :

=over

=item A

A description of the first I<association end>, which is composed of

=over

=item table

An instance of L<DBIx::DataModel::Meta::Source::Table>.

=item role

The role name of that source within the association.
A Perl method of the same name will be defined in the
remote source (the other end of the association).
Besides, the role name is also used when building
joins through 

  $schema->join(qw/FirstTable role1 role2 .../)

One of the role names in the association can be
I<anonymous>, which is expressed by C<undef>, 
by an empty string, or a string containing C<'0'>, 
C<'none'>, or C<'---'>. If anonymous, there
will be no Perl method and no possibility to join in that
direction, so it defines a unidirectional association.
Two anonymous roles in both directions are forbidden (because
in that case the association would be useless).

If several associations attempt to define the same role name in
the same table, an exception is generated.

=item multiplicity

The multiplicity specification, i.e. the minimum and maximum 
number of occurrences of that association end, for any given
instance of the other end (if not clear, see UML textbooks).

The multiplicity can be expressed either as an 
arrayref C<< [$min, $max] >>, or as a string C<"$min..$max">.
The C<$max> can be C<'*'> or C<'n'>, which is interpreted
as the maximum integer value. If expressed as a string,
a mere C<'*'> is interpreted as C<'0..*'>, and a mere
C<'1'> is interpreted as C<'1..1'>.

Numbers different from C<0>, C<1> or C<*> may be given
as multiplicity bounds, but this will be just documentary :
technically, all that matters is

=over

=item *

whether the lower bound is 0 or more (if 0, generated
joins will be left joins, otherwise inner joins)

=item *

whether the upper bound is 1 or more (if 1, the associated
method returns a single object, otherwise it returns an arrayref)

=back


=item join_cols

An arrayref of columns that participate in the database join,
for this side of the association. The full database join will
be built by creating a C<LEFT|INNER JOIN ... ON ..> clause in
which the left-hand and right-hand sides of the C<ON> subclause
come from the C<join_cols> of both association ends.

This argument is optional: if absent, it will be filled
by default by taking the primary key of the table with minimum
multiplicity 1, for both sides of the association.
This default behaviour is convenient for data models where
primary keys and foreign keys are identical (for example C<dpt_id> both
as primary key in C<Department> and as foreign key in C<Activity>). 
Some data models have a different policy, where primary keys are 
always named the same (for example C<id>), and foreign keys
are named after the related table name : in such models, the default
does not work, and you have to specify the join columns
explicitly.

If the association is many-to-many (i.e. if the maximum
multiplicity is greater than 1 on both sides), then 
C<join_cols> takes a special meaning : it no longer
represents database columns, but rather represents 
two role names (in the sense just defined above) to follow
for reaching the remote end of the association.
Therefore C<join_cols> must contain exactly 2 items in that case :
the path to the intermediate table, and the path from the intermediate
table to the remote end. Here is again the example from 
L<DBIx::DataModel/SYNOPSIS> : 

  My::Schema->define_association(
    kind => 'Association',
    A    => {
      table        => My::Schema::Department->metadm,
      role         => 'departments',
      multiplicity => '*',
      join_cols    => [qw/activities department/],
    },
    B    => {
      table        => My::Schema::Employee->metadm,
      role         => 'employees',
      multiplicity => '*',
      join_cols    => [qw/activities employee/],
    },
  );

=back

=item B

A description of the second I<association end>, following exactly the 
same principles as for the C<'A'> end.

=item name

Optional name for the association (otherwise an implicit name
will be built by default from the concatenation of the role names).

=item kind

A string describing the association kind, i.e. one of :
C<Association>, C<Aggregation> or C<Composition>.
See L</Composition()> below for the additional semantics associated
with compositions.

=back

The association also creates instances of 
L<DBIx::DataModel::Meta::Path> for representing the 
directional paths between those sources.

Only binary associations can be declared; however, it is possible
to define methods joining three or more tables : see
L</define_navigation_method()>.


=head4 Special semantics for compositions


Compositions are associations with some additional semantics. In UML
class diagrams, compositions are pictured with a black diamond on one
side : this side will be called the I<composite> class, while the
other side will be called the I<component> class. In
C<DBIx::DataModel>, the diamond (the composite class) corresponds to
the C<A> association end, and the component class corresponds to the
C<B> end, so the order is important (while for plain associations the
order makes no difference).

The UML intended meaning of a composition is that objects of the
component classes cannot exist outside of their composite class. Within
C<DBIx::DataModel>, the additional semantics for compositions is to
support cascaded insertions and deletions, and auto-expansion; so the
special semantis attached to kind C<Composition> is :

=over

=item *

the multiplicity must be 1-to-n

=item * 

the C<'B'> end of the association (the "component" part) must not 
be component of another association (it can only be component of one
single composite table).

=item *

this association can be used for auto-expanding the composite object
(i.e. automatically fetching all component parts from the database)
-- see L<DBIx::DataModel::Source/expand>
and L</auto_expand>

=item *

this association can be used for cascaded inserts like

  $source->insert({
    column1 => $val1,
    ...
    $component_name1 => [{$sub_object1}, ...],
    ...
   })

see L</insert>

The main record will be inserted in the composite class, and within
the same transaction, subrecords will be inserted into the 
component classes, with foreign keys automatically filled with
appropriate values.


=item *

this association can be used for cascaded deletes : 
the argument to a C<delete> may contain lists of component records to
be deleted together with the main record of the composite class.


=back

=head3 define_type()

  $meta_schema->define_type(
     name     => $type_name,
     handlers => {
       $handler_name_1 => sub { ... },
       $handler_name_2 => sub { ... },
       ...
       },
   );

This declares a I<type>, which is just a hashref of handler names and
handler bodies (coderefs). The type can then be applied to some
columns in some tables; this is usually done in the Table declaration
(C<column_types> argument), or can be applied later through the
L</define_column_type> method.

Handlers receive the column value as usual through C<< $_[0] >>.
If the value is to be modified (for example for scalar
conversions or while inflating values into objects), 
the result should be put back into C<< $_[0] >>.
In addition to the column value, other info is passed to the
handler :

  $handler_body = sub {
    my ($column_value, $obj, $column_name, $handler_name) = @_;
    my $new_val = $obj->compute_new_val($column_value, ...);
    $column_value = $new_val; # WRONG : will be a no-op
    $_[0] = $new_val;         # OK    : value is converted
  }

The second argument C<< $obj >> is the object from where 
C<< $column_value >> was taken -- most probably an instance 
of a Table or Join class.  Use this if you need to read some contextual
information, but avoid modifying C<< $obj >> : you would most
probably get unexpected results, since the collection of 
available columns may vary from one call to the other.

Other arguments C<< $column_name >> and
C<< $handler_name >> are obvious.

Handler names B<from_DB> and B<to_DB> have a special
meaning : they are called automatically just after reading data from
the database, or just before writing into the database.
Handler name B<validate> is used by the method
L</"has_invalid_columns()">.

The L<DBIx::DataModel/SYNOPSIS> shows some examples for types 
"Date", "Percent" and "Multivalue".

A C<Type> does I<not> create a Perl class.


=head3 define_join()

This method builds or retrieves a pre-existing subclass of 
L<DBIx::DataModel::Source::Join>, and returns the associated
meta-object (an instance of L<DBIx::DataModel::Meta::Source::Join>).

  my $meta_join = $meta_schema->define_join($table, $path1, $path2, ..);

The join starts from a given table class and
follows one or several associations through their path names;
the resulting SQL request contains parameters
automatically inferred from the associations. So for example

  $meta_schema->define_join(qw/Department activities employee/);

is more or less equivalent to 

  my $sql = <<_EOSQL_
  Department 
    LEFT OUTER JOIN Activity ON Department.dpt_id = Activity.dpt_id
    LEFT OUTER JOIN Employee ON Activity.emp_id   = Employee.emp_id
  _EOSQL_
  
  $schema->View("Department=>activities=>employee", '*', $sql, 
                qw/Department Activity Employee/);

For each pair of tables, the kind of join is chosen according to the
multiplicity declared with that role in the association : if the
minimum multiplicity is 0, the join will be LEFT OUTER JOIN; otherwise
it will be a usual inner join (exception if 
C<< $meta_schema->sql_no_inner_after_left_join >> is true : after a first
left join, all remaining tables are also connected through additional
left joins). The default kind of join chosen by this rule may be
overriden by inserting intermediate connectors in the list, namely 
C<< '<=>' >> for inner joins and C<< '=>' >> for left joins.
So for example

  $meta_schema->define_join(qw/Department <=> activities <=> employee/);

becomes equivalent to 

  my $sql = <<_EOSQL_
  Department 
    INNER JOIN Activity ON Department.dpt_id = Activity.dpt_id
    INNER JOIN Employee ON Activity.emp_id   = Employee.emp_id
  _EOSQL_

Table names in the SQL statement correspond to what was defined
in the C<< $meta_schema->Table(...) >> declarations. However, tables may 
be aliased using C<|> as a separator :

  $meta_schema->define_join(qw/Department|dpt activities|act employee|emp/)
              ->select(-columns => qw/dpt.name act.d_begin emp.lastname/,
                       -where   => {"dpt.dpt_name" => {-like => "A%"}});

which generates

  SELECT dpt.name, act.d_begin, emp.lastname
  FROM Department AS dpt
    LEFT OUTER JOIN Activity AS act ON dpt.dpt_id = act.dpt_id
    LEFT OUTER JOIN Employee AS emp ON act.emp_id = emp.emp_id
  WHERE dtp.dpt_name like 'A%'

All tables participating in a C<join> are stacked,
and further roles are found by walking up the stack. So in

  ..->define_join(qw/FirstTable path1 path2 path3/)

we must find a C<path1> in C<FirstTable>, from which we 
know what will be the C<Table2>. Then, we must find 
a C<path2> in in C<Table2>, or otherwise in C<FirstTable>,
in order to know C<Table3>. In turn, C<path3> must be 
found either in C<Table3>, or in C<Table2>, or in C<FirstTable>, etc.
To resolve ambiguities, prefix the path by the name or alias of the
targeted source, such as :

  ..->define_join(qw/FirstTable path1|p1 
                     FirstTable.path2 
                     p1.path3|p3 
                     path2.path4/)

The name of the resulting join class will be composed by concatenating
table, connectors and path names, including optional aliases. If the
same sequence of table and paths was already encountered before, the
Perl class already exists, and its corresponding meta-object is returned;
otherwise, a new Perl class is created together with its meta-object.

The main purpose of C<define_join> is to gain efficiency in
interacting with the database. If we write

  foreach my $dpt (@{$schema->table('Department')->select}) {
    foreach my $act ($dpt->activities) {
      my $emp = $act->employee;
      printf "%s works in %s since %s\n", 
         $emp->{lastname}, $dpt->{dpt_name}, $act->{d_begin};
    }
  }

many database calls are generated behind the scene, in the loops that 
call the C<activities> and C<employee> methods.
Instead we could write 

  my $join = $meta_schema->define_join(qw/Department activities employee/);
  foreach my $row (@{$join->select}) {
    printf "%s works in %s since %s\n", 
      $row->{lastname}, $row->{dpt_name}, $row->{d_begin};
  }

which generates one single call to the database.

=head2 Meta-table methods

=head3 define_navigation_method()

  $meta_table->define_navigation_method(
    $meth_name => qw/role1 role2 .../, 
    \%optional_select_args,
  );

Inserts into C<< $meta_source->class >> a new method named C<$meth_name>,
that will automatically call L</"define_join()"> and
then L<select()|/"select">, passing C<%optional_select_args> to the 
C<select> call. This is useful for joining several tables at once, so for
example with 

  $meta_schema->table('Department')->define_navigation_method(
    employees => qw/activities employee/
  );

we can then write 

  my $empl_ref = $some_dept->employees(-where   => {gender => 'F'},
                                       -columns => [qw/firstname lastname]);

This method is used internally to implement I<many-to-many> associations;
so if you have only two roles to follow, you would probably be better
off by defining a many-to-many association, which is a more abstract notion.
Direct calls to C<define_navigation_method> are still useful if you want
to follow three or more roles at once.

The last argument to C<define_navigation_method> is an optional 
hashref; if present, the hash will be passed as initial
argument to the C<select> call.


=head3 define_column_type()

  $meta_table->define_column_type($type_name, @column_names);

Registers type C<$type_name> to be applied to columns with
name in C<@column_names>, within the target C<$meta_table>.


=head3 define_column_handlers()

  $meta_table->define_column_handlers($column_name, 
    $handler_name_1 => sub { ... },
    ...
  );

Registers handlers to be applied to C<$column_name>,
within the target C<$meta_table>. The main purpose of this method
is for implementing the higher-level L</define_column_type()> method; 
however it can also be called directly, without the need for defining
a type.


=head3 define_auto_expand()

  $meta_table->define_auto_expand(@component_names);

Generates an L</"auto_expand()"> method for the class, that 
will autoexpand on the roles listed (i.e. will call
the appropriate method and store the result
in a local slot within the object). 
In other words, the object knows how to expand itself,
fetching information from associated tables, in order
to build a data tree in memory.
Only component names declared through L<Composition()|/"Composition">
may be auto-expanded.

Be careful about performance issues: when an object uses
auto-expansion through a call to L</"auto_expand()">, every
auto-expanded role will generate an additional call to the
database. This might be slow, especially if there are recursive
auto-expansions; so in some cases it will be more appropriate to
flatten the tree and use database joins, typically through the
L<join()|/"Schema::join()"> method.



=head1 SCHEMAS

A C<$schema> is an I<instance> of a I<subclass> of 
L<DBIx::DataModel::Schema>. The subclass holds a reference
to a C<$meta_schema> where all information about tables, 
associations, etc. is kept. Each instance of that subclass
holds information about the database connection, the debugging
status, etc.

A schema subclass can have several instances; in that case, the application
runs in I<multi-schema> mode. However, multi-schema mode is only useful
in some special situations, like for example when transferring data between
several databases; in most common cases, a I<single-schema> mode is 
enough. 

Single-schema mode is activated by default, which means that
all method calls can be performed directly on the subclass; the subclass
will manage a I<singleton> instance, and will delegate calls to that
singleton. This is the recommended way to work with C<DBIx::DataModel>,
because it uses less memory, and simplifies the application code (there 
is no need to pass a C<$schema> reference around between all modules).

If you need it, multi-schema mode is activated by calling the C<new()>
method, as many times as you need. Once this mode is activated, it is
not possible to go back to single-schema mode. Furthermore, multi-schema
mode should be activated before the singleton has been created, i.e. before
any call to the class methods below.

Because of this duality, all methods below, described
as C<< $schema->some_method(...) >>, can be called either as 
I<class> methods (single-schema mode), or as I<instance> methods
(multi-schema mode).


=head2 Creating and manipulating schema state

=head3 Schema::new()

  my $schema = $schema_subclass->new(%options);

As just explained, this activates multi-schema mode, and returns
a new schema instance. C<%options> may contain some initial settings
for C<dbh>, C<debug>, etc.; this is the same as creating the schema
first, and then calling the setter methods below
C<< $schema->dbh(...) >>, C<< $schema->debug(...) >>, etc.


=head3 singleton()

  my $schema = $schema_subclass->singleton;

When in single-schema mode, returns the singleton instance
associated with this C<$schema_subclass>. When in multi-schema
mode (i.e. after C<Schema::new()> has been called at least once);
raises an error.

Client code rarely needs to call C<singleton()> explicitly; when
in single-schema mode, method calls are usually expressed as
class methods, and the singleton is retrieved automatically.

=head3 dbh()

  my $dbh = DBI::connect(...);
  $schema->dbh($dbh, %options);        # set
  $schema->dbh([$dbh, %options]);      # set, alternative syntax

  my $dbh             = $schema->dbh;  # get back just the dbh
  my ($dbh, %options) = $schema->dbh;  # get back all

Returns or sets the handle to a DBI database handle (see L<DBI>).  The
C<$dbh> handle must have its C<< RaiseError >> property set to a true
value.

In C<%options> you may pass any key-value pairs, and retrieve
them later by calling C<dbh> in a list context,
which may be useful for holding driver-specific information.
C<DBIx::DataModel> will only inspect those options for knowing
how to retrieve database-generated keys (see the L</"insert"> method 
below).

Calls to the C<dbh> method for changing the database handle are
allowed at any time, except when a transaction is in course.  However,
a nested transaction may temporarily change the database handle by
supplying it as argument to the L</do_transaction> method.

To unset the database handle, call C<< $schema->dbh(undef) >>.

=head3 debug()

  $schema->debug(1);             # will warn for each SQL statement
  $schema->debug($debug_object); # will call $debug_object->debug($sql)
  $schema->debug(undef);         # turn off debugging

Debug mode is useful for seeing SQL statements generated 
by C<DBIx::DataModel>. 
Enabling debugging with a C<$debug_object>
will typically be useful in conjunction with something like 
L<Log::Log4perl|Log::Log4perl> or 
L<Log::Dispatch|Log::Dispatch>.
Otherwise, enabling debugging with just any true scalar
will print SQL statements on STDERR.

There is also another way to see the SQL code for one particular statement :

  my $spy_sql = sub {my ($sql, @bind) = @_;
                     print STDERR join "\n", $sql, @bind;
                     return ($sql, @bind);};
  
  my $result = $source->select(-columns   => \@columns,
                               -where     => \%criteria,
                               -post_SQL  => $spy_sql);

=head3 sql_abstract()

  $schema->sql_abstract($an_SQL_Abstract_More_instance); # set
  my $sqlam = $schema->sql_abstract;                     # get

Sets or retrieves the instance of L<SQL::Abstract::More> used 
by this C<$schema>. If the client code does not set it explictly,
an instance wil be implicitly created.

=head3 dbi_prepare_method()

  $schema->dbi_prepare_method($method);      # set
  my $method = $schema->dbi_prepare_method;  # get

Sets or retrieves the method sent to L<DBI> for preparing SQL statements.
The default is C<"prepare">; it can be set to C<"prepare_cached"> instead.


=head3 placeholder_prefix()

  $schema->placeholder_prefix($prefix);      # set
  my $prefix = $schema->placeholder_prefix;  # get

Sets or retrieves the prefix string to recognize "named placeholders"
within a statement. That prefix should never match any regular data
encountered in your application; the default is C<'?:'>.

=head3 select_implicitly_for()

  $schema->select_implicitly_for($string);      # set
  my $string = $schema->select_implicitly_for;  # get

Sets  or retrieves a default value for the C<-for> argument to 
L<select()|/"select">. 

This default value is not set for statements with 
C<< -result_as => 'subquery' >> (because the FOR clause in an 
SQL statement only makes sense at the top level, not in a subquery).


=head3 localize_state()

  {
    my $scope_guard = $schema->localize_state(@schema_members_to_localize);

    ... # do some work, possibly change state
  
  } # $scope_guard out of scope : previous state of $schema is restored 


Applies dynamic scoping to a C<$schema>, like Perl's
C<local> construct (see L<perlsub/"Temporary Values via local">).  Here
however, since C<local> is not perfectly suited for private class
data, we provide a solution which is similar in spirit, but slightly
different in implementation. The C<localize_state> method 
internally takes a copy of the current state, and returns
a handle to it. The handle should be stored in a C<my> variable;
when that variable goes out of scope (at block exit), then
the previous state is restored.

The optional argument C<@schema_members_to_localize> specifies
precisely which schema members should be localized. 
When called with an empty list, the default is : 
C<dbh>, C<dbh_options>, C<debug>, 
C<select_implicitly_for>, C<dbi_prepare_method>. 


=head2 Creating "connected sources"

Data is stored in I<data sources> that are accessed through a given
I<schema>; therefore, before issuing data manipulation statements, we
need to establish a relationship between a data source and a schema :
this is the job of the two methods below. Both create a
L<DBIx::DataModel::ConnectedSource> instance.  That object can then
perform data selection, update or deletion, as described in the next
chapter.

=head3 Schema::table()

  my $source = $schema->table($table_name);
  my $result = $source->select(...); # or ->insert(..), ->update(..), etc.

This is the recommended way to reach a table from a schema.
The C<$table_name> is the Perl name that was declared in L</define_table()>,
not the database name.

The method name is (intentionally!) misleading : instead of returning
a I<table> object or I<meta_table> object, it returns a I<connected_source>;
however that object delegates many methods to the table and therefore
can mimic it (actually, it works like a kind of proxy). 

When in single-schema mode, the C<table()> call can be replaced by a
direct call to the table class: so for example, if we have a schema
class C<HR> containing a table C<Employee>, the following calls are
equivalent :

  $result = HR::Employee->select(...)
  $result = HR->table('Employee')->select(...)
  $result = HR->singleton->table('Employee')->select(...)

In the first syntax above, the C<HR::Employee> subclass will implicitly
reach for its schema singleton and for its meta-table, and 
will implicitly create a statement.

When in multi-schema mode, the only way is through the C<table()> method :

  my $schema1 = HR->new(dbh => dbh1);
  my $schema2 = HR->new(dbh => dbh2);
  $result1 = $schema1->table('Employee')->select(...);
  $result2 = $schema2->table('Employee')->select(...);

This C<Schema::table()> method should not be confused with
the meta-schema navigation method L</Meta::Schema::table()>, described 
at the end of this document.

=head3 Schema::join()

  my $source = $schema->join($table, $path1, $path2, ...);
  my $result = $source->select(...);

This method calls L</define_join()> to define or retrieve a join
subclass; then it creates a fresh I<connected_source> related to that 
join datasource and to the C<$schema>. From that object, data
can be retrieved through the L</select()> method.

This C<Schema::join()> method should not be confused with
the meta-schema navigation method L</Meta::Schema::join()>, described 
at the end of this document, which is just an accessor method.

Besides, a derivative of the present method is defined 
in L</Table::join()>.


=head2 Transactions

=head3 do_transaction()

  my $coderef = sub {$table1->insert(...); $table2->update(...); ...};
  $schema->do_transaction($coderef);

Evaluates the code within a transaction. In case of failure, the
transaction is rolled back, and an exception is raised, with a message
containing the initial error and the status of the rollback (because
the rollback itself may also fail). If you need finer details,
you can treat the exception as an object with two methods C<initial_error>
and C<rollback_errors>:

    eval {$schema->do_transaction($coderef); 1}
    or do {my $err = $@;
           explain($err->initial_error, $err->rollback_errors)};

Usually the coderef passed as argument will be a
closure that may refer to variables local to the environment where
the closure was created.

Nested calls to C<do_transaction> are supported : only 
the top-level call will actually initiate and then
commit the transaction, while an exception at any level
will abort and rollback the whole thing.

If the nested transaction needs to temporarily work on a different 
database handle, it may do so by supplying the dbh and its
options as additional arguments :

  $schema->do_transaction($coderef, $new_dbh, %new_dbh_options);

When called in this form, C<do_transaction> will temporarily
set the dbh to the supplied value, and then return to the 
previous value when the nested transaction is finished.
Commits on all involved database handles are delayed until
the top-level transaction is finished.

There is no support (yet!) for nested transactions with
intermediate savepoints.

=head2 Schema utility methods

=head3 Schema::metadm()

Returns the meta-schema instance associated with the invocant.

=head3 unbless()

  $schema->unbless($obj1, $obj2, ...);

Recursively applies L<Acme::Damn/damn> to remove all class information 
from its arguments : these become plain Perl hashrefs, arrayrefs or scalars.

This may be useful if the datatree returned by a C<select(..) / expand(..) >
needs to be exported through an external module that only considers
plain datastructures; this is the case for example with L<YAML::Syck/Dump>.




=head2 Schema class methods, delegated to the meta-schema

Front-end data definition methods listed in 
L</"META-SCHEMA DEFINITION"> can be called on the schema
class, instead of the meta-schema; in other words, the 
following methods can be called on the schema class :

=over

=item L</Table()>

=item L</View()>

=item L</Association()>

=item L</Composition()>

=item L</Type()>

=back


=head1 CONNECTED SOURCES

=head2 Principle

A I<connected source> is basically just a pair of references to a
C<$meta_source> and to a C<$schema>. Such objects are created
explicitly through methods L</Schema::table()> or L</Schema::join()>;
they are also created implicitly when, in single-schema mode, class
methods are called on table classes. The constructor is not documented
because client code is not expected to call it directly; instead,
client code should use the C<Schema> methods just mentioned.


=head2 Accessors

=head3 schema()

Accessor for the connected source's schema.

=head3 source()

Accessor for the connected source's meta-source.

=head3 metadm()

Synonym for C<source()>.
This is for convenience, because in something like

  my $meta_table = $schema->table($table_name)->metadm;

one naturally expects to get the meta-object associated with table
C<$table_name>, forgetting the fact that the subexpression 
C<<$schema->table($table_name) >> is a connected source and not a table!


=head2 Data retrieval

=head3 ConnectedSource::select()

Proxy method to L<Statement::select()|/"select()">

=head3 fetch()

  my $record = $connected_source->fetch(@key_values, \%options);

Fetches a single record, from its primary key value
(on one or several columns). This method is just syntactic sugar for 

  my $record = $connected_source->select(-fetch => \@key_values, %options);

(see below). C<%options> may specify arguments to L</select()>, 
like C<-for>, C<-pre_exec>, C<-post_exec>, etc. 


=head3 fetch_cached()

  my $record = $connected_source->fetch_cached(@key_values, \%options);

Like C<fetch>, except that the result is stored in a cache,
and further calls to the same methods with the same
parameters will return the cached record instead of going
back to the database. The cache does not know about any updates 
to the database, so this is mainly useful for readonly data.

The cache is stored internally in
C<< $meta_source->{fetch_cached}{$dbh_addr}{$freeze_args} >>
(where C<$dbh_addr> is C<< Scalar::Util::refaddr(MyTable->dbh) >>
and C<$freeze_args> is C<< Storable::freeze(@keyValues, \%options) >>).
If needed, client code may use this information to clear the cache or tie 
it to a more sophisticated caching module.


=head3 ConnectedSource::join()

  my $join_stmt = $connected_source->join(qw/path1 path2 .../);
  $join_stmt->prepare();

Returns a statement that will select a collection of data rows from
tables associated with the current meta-source, performing the
appropriate joins.  Internally this is implemented throught the
C</define_join()> method, with an additional C<-where> criteria to
constrain on the primary key(s) of the meta-source.  That statement
cannot be executed yet, because the values of the primary key are not
known until we have an row of that source; but the statement can
already be L<prepare|/prepare()>d.  Later on, we can
L<execute|/execute()> the statement by binding it to an instance of
the C<$connected_source> :

   my $obj          = $connected_source->fetch(...);
   my $related_rows = $join_stmt->execute($obj)->all;

This is especially useful for loop efficiency :

  my $join_stmt = $connected_source->join(qw/path1 path2 .../);
  $join_stmt->prepare();
  my $list = $connected_source->select(...);
  foreach my $row (@$list) {
    my $related_rows = $join_stmt->execute($row)->all;
    # ... work with $related_rows
  } 

=head2 Data manipulation

=head3 insert()

  my @ids = $connected_source->insert(
    {col1 => $val1, col2 => $val2, ...}, 
    {...},
    %options,
  );
  # or
  my @ids = $connected_source->insert(
    [qw/ col1   col2   .../],
    [    $val1, $val2, ... ],
    ...
  );

Inserts a collection of rows into the database, given
either as a list of hashrefs, or as a first arrayref containing
the column names, followed by a list of arrayrefs containing
values for each row to be inserted.

In either form, the method applies the C<to_DB> handlers, removes the
C<no_update> columns, and then inserts the new records into the
database. 

Primary key column(s) should of course be present 
in the supplied hashrefs, unless the the key is auto-generated
by the database (see below). 

Each hashref will be blessed into the C<$table> class, and
will be inserted through the internal 
L<_singleInsert()|DBIx::DataModel::Doc::Internals/"_singleInsert"> method.
The default implementation of this method should be good enough
for most common uses, but you may want to refine it in your
table classes if you need some fancy handling on primary keys
(like for example computing a random key and checking whether
that key is free; see L<DBIx::DataModel::Doc::Internals> for an example).
The default implementation uses the following
algorithm to retrieve keys auto-generated by the database :

=over

=item *

if a dbh option called C<returning_through> is found (see options 
passed to the L</"dbh"> method), the method automatically adds 
a C<-returning> clause for retrieving value(s) from the primary
key column(s). The way to retrieve such values depends on 
C<returning_through>: when 'FETCH', it performs an additional 
call to C<< $sth->fetchrow_array >>; when 'INOUT', it binds 
inout parameters into the statement. When setting the database handle
through the L</dbh> method, the C<returning_through> option is automatically
set to 'FETCH' if it is a Pg driver, or automatically set to 'INOUT'
if it is an Oracle driver.

=item *

if a dbh option called C<last_insert_id> is found, this is
taken as a callback function, which gets called as

  $dbh_options{last_insert_id}->($dbh, $table_name, $column_name)

=item *

if dbh options called C<catalog> and/or C<schema> are found,
C<DBIx::DataModel> will call 

  $dbh->last_insert_id($dbh_options{catalog}, $dbh_options{schema},
                       $table_name, $column_name)

=item *

otherwise, C<DBIx::DataModel> will call 

  $dbh->last_insert_id(undef, undef, undef, undef)

=back

=head4 Cascaded insert

If the table is a composite class (see L<Composition()|/"Composition">
above), then the component parts may be supplied within the hashref,
with keys equal to the role names, and values given as arrayrefs of
sub-hashrefs; then these will be inserted into the database, at the
same time as the main record, with join values automatically filled
in (cascaded insert).  For example :

   HR::Employee->insert({firstname  => "Johann Sebastian",  
                         lastname   => "Bach",
                         activities => [{d_begin  => '01.01.1695',
        			         d_end    => '18.07.1750',
	        		         dpt_code => 'CPT'}]});

=head4 Insert options

The C<insert()> call may take a list of B<%options> 
specified at the I<end> of the argument list (notice they are
I<not> given as a hashref, but as a mere hash, or list of pairs).
Actually the only option currently supported is 
B<-returning>,  :

=over

=item *

if the C<-returning> option is set to an empty hashref, the return
value will be a list of hashrefs (one for each inserted record),
containing the column name(s) and value(s) of the primary key for that
record, and possibly containing subhashes or subarrays for other
records created through cascaded inserts. For example:

  my @result = HR->table('Employee'>
                 ->insert({..., activities => [{...}, ...]},
                          ...,
                          -returning => {});
  my $prim_key_first_emp = $result[0]{emp_id};
  my $prim_key_first_act = $result[0]{activities}[0]{act_id};

=item *

if the C<-returning> option is set to any other value, that value is passed
to L<SQL::Abstract::More/insert> and finally to the SQL level 
(INSERT ... RETURNING ...);  whatever is returned from the 
database for each single record gets flattened into a single
list transmitted back to the caller.

  my @result = $statement->insert({...}, ..., 
                                  -returning => $scalar_or_arrayref);


=item *

if the C<-returning> option is absent, values returned by calls to
L<_singleInsert()|DBIx::DataModel::Doc::Internals/"_singleInsert">
are collected into a flattened
array, and then returned by C<insert()>; usually, these are the
primary keys of the inserted records.  If this array contains several
values, and C<insert()> was called from a scalar context, a warning is
issued.

=back

=head3 update()

  $connected_source->update(-set   => {col1 => $val1, ...},
                            -where => \%condition);
  $connected_source->update({col1 => $val1, ...});
  $connected_source->update(@primary_key, {col1 => $val1, ...});

Generates a request to the database to update one or several
records. The API for this method accepts several syntaxes :

=over 

=item *

the first syntax with C<-set> and C<-where> keywords
closely reflects the SQL syntax of shape 

  UPDATE table SET col1='val1', col2='val2', ... WHERE ...

This is mostly used for updating several records simultaneously (bulk
update). 

=item * 

the second syntax is used for updating a single record, passed as a
hashref; it does not matter if this hashref is blessed or not, because
a copy will be blessed into the class of the invocant. 
A C<-where> clause will be automatically generated by extracting the
primary key column(s) from the record; then the remaining columns are
treated as the columns to update. 

=item * 

the third syntax with C<< @primary_key >> is an alternate way to
supply the values for the primary key (it may be more convenient
because you don't need to repeat the name of primary key columns). So
if C<emp_id> is the primary key of table C<Employee>, then the
following are equivalent :

  HR->table('Employee')->update({emp_id  => $eid, 
                                 address => $new_addr, 
                                 phone   => $new_phone});
  HR->table('Employee')->update($eid => {address => $new_addr, 
                                         phone   => $new_phone});

=back

Yet another syntax is to directly call C<update()> on a data row,
without any argument : see L</Table::update()> below.

Before calling the database, the
C<to_DB> handlers are applied, the C<no_update_columns> are
removed, and the C<auto_update_columns> are inserted.

The C<update> method only updates the columns received
as arguments : it knows nothing about other columns that may sit
in the database. Therefore if you have two concurrent clients
doing

  (client1)  ...->update($id, {c1 => $v1, c2 => $v2});
  (client2)  ...->update($id, {c3 => $v3, c4 => $v4, c5 => $v5});

the final state of record C<$id> in the database is guaranteed to
reflect changes from both clients, because the sets C<('c1', 'c2')>
and C<('c3', 'c4', 'c5')> of updated columns are disjoint.

=head3 delete()

  $connected_source->delete(-where => \%condition);
  $connected_source->delete({col1 => $val1, ...});
  $connected_source->delete(@primary_key);

Generates a request to the database to delete one or several
records. The API for this method accepts several syntaxes :

=over 

=item *

the first syntax with the C<-where> keyword
closely reflects the SQL syntax of shape 

  DELETE FROM table WHERE ...

This is mostly used for deleting several records simultaneously (bulk
delete). 

=item * 

the second syntax is used for deleting a single
record. A C<-where> clause will be automatically generated
by extracting the primary key column(s) from the record.

If the source is a composite class (see
L<Composition()|/"Composition"> above), and if the record contains
references to lists of component parts, then those will be recursively
deleted together with the main object (cascaded delete). However, if
there are other component parts in the database, not referenced in the
hashref, then those will not be automatically deleted : in other
words, the C<delete> method does not go by itself to the database to
find all component parts (this is the job of the client
code, or sometimes of the database itself).

=item * 

the third syntax with C<< @primary_key >> is an alternate way to
supply the values for the primary key (it may be more convenient
because you don't need to repeat the name of primary key columns).
Note that C<< $statement->delete(11, 22) >>
does not mean "delete records with keys 11 and 22", but rather
"delete record having primary key (11, 22)"; in other words,
with this syntax you only delete one record at a time. 

=back

Yet another syntax is to directly call C<delete()> on a data row,
without any argument : see L</Table::delete()> below.



=head2 Utility methods

=head3 ConnectedSource::bless_from_DB()

Proxy method to L<Statement::bless_from_DB()|/"bless_from_DB()">.


=head1 STATEMENTS

A I<statement> object encapsulates a SELECT request to the database.  It
starts by assembling various pieces of information : the datasource to
query, the database connection, the various clauses for generating the
SQL request.  Once everything is ready, the database request is
issued, and the results are collected. These steps may occur either
all at once (which is the most frequent case), or through several
calls to intermediate methods; see for example the methods
L</refine()>, L</prepare()>, L</execute()> below.

By default, statements are created as instances of 
L<DBIx::DataModel::Statement|DBIx::DataModel::Statement>; 
but the schema may define another L</statement_class()> 
for application-specific purposes : for instance the
L<DBIx::DataModel::Statement::JDBC|DBIx::DataModel::Statement::JDBC>
subclass in this distribution adds some methods specific to
L<DBD::JDBC|DBD::JDBC> data sources, allowing to call
JDBC methods C<getMemberCount()>, C<absolute()>, C<relative()>, etc.


=head2 Data selection

=head3 select()

  $result = $statement->select(
     -columns       => \@columns, 
       # OR : -columns => [-DISTINCT => @columns],
     -where         => \%where,
       # OR : -fetch => $key,
       # OR : -fetch => \@key,
     -group_by      => \@groupings,
     -having        => \%criteria,
     -order_by      => \@order,
     -for           => $purpose,
     -post_SQL      => sub {...},
     -pre_exec      => sub {...},
     -post_exec     => sub {...},
     -post_bless    => sub {...},
     -prepare_attrs => \%attrs,
     -limit         => $limit,
     -offset        => $offset,
     -page_size     => $page_size,
     -page_index    => $page_index,
     -column_types  => \%column_types,
     -result_as     => 'rows'      || 'firstrow' 
                    || 'hashref'   || [hashref => @cols]
                    || 'sth'       || 'sql' 
                    || 'subquery'  || 'flat_arrayref' 
                    || 'statement' || 'fast_statement'
  );
  

Calls L</refine()> to process arguments (if any), and returns a result
as specified by the C<-result_as> argument (see below).
If necessary, intermediate steps are automatically
performed, like calling L</sqlize()>, L</prepare()> and
L</execute()>.

Arguments are all optional and are passed by name :

=over

=item C<< -columns => \@columns >> 

C<< \@columns >>  is a reference to an array
of SQL column specifications (i.e. column names, 
C<*> or C<table.*>, functions, etc.).

Initial words in C<@columns> that start with a hyphen 
are treated as verbatim SQL : in particular, 
C<< -columns => [-DISTINCT => qw/col1 col2 .../] >>
will yield 

  SELECT DISTINCT col1, col2, ... FROM ...

A '|' in a column is translated into an 'AS' clause, according
to the current SQL dialect in L<SQL::Abstract::More> : this is convenient when
using perl C<< qw/.../ >> operator for columns, as in

  -columns => [ qw/table1.longColumn|t1lc table2.longColumn|t2lc/ ]

Column aliasing should be avoided on key columns (either primary or 
foreign keys), because role methods will no longer be able
to navigate through the joins (currently C<DBIx::DataModel> is 
not clever enough to rename its internal join constraints according
to column aliases). Aliasing on non-key columns is OK, and 
column handlers will operate properly on aliased columns.

The argument to C<-columns> can also be a string instead of 
an arrayref, like for example
C<< "c1 AS foobar, MAX(c2) AS m_c2, COUNT(c3) AS n_c3" >>;
however this is mainly for backwards compatibility. The 
recommended way is to use the arrayref notation as explained above :

  -columns => [ qw/  c1|foobar   MAX(c2)|m_c2   COUNT(c3)|n_c3  / ]

If omitted, C<< \@columns >> takes the default, which is
usually '*', unless modified through the C<default_columns> argument
to  L</define_table()>.

No verification is done on the list of retrieved C<< \@columns >>; in
particular, the list must not always include primary or foreign keys
--- but if this is the case, later attempts to perform joins or
updates will obviously fail.


=item C<< -where => \%where >> 

C<< \%where >> is a reference to a hash or array of 
criteria that will be translated into SQL clauses. In most cases, this
will just be something like C<< {col1 => 'val1', col2 => 'val2'} >>;
see L<SQL::Abstract::select|SQL::Abstract/select> for 
 detailed description of the
structure of that hash or array. It can also be
a plain SQL string like C<< "col1 IN (3, 5, 7, 11) OR col2 IS NOT NULL" >>.


=item C<< -fetch => \@columns >>

equivalent to 

  ...->select(-where => {<primary_key_column0> => $columns[0],
                         <primary_key_column1> => $columns[1], ...},
              -result_as => "firstrow")

If the primary key ranges on one single column (which is the most
frequent case), then the argument to C<-fetch> can also be
a single scalar value :

  ...->select(-fetch => $key)

When calling a table directly, the C<< select(-fetch => ..) >> syntax 
is awkward; you will most certainly prefer the syntactic
sugar offered by the L<fetch()|"fetch()"> method :

  $table->fetch(@key)

However, the C<-fetch> notation is useful when walking through
association roles :

  $employee->activities(-fetch => $act_id)

This example will generate the following SQL 

  SELECT * FROM activity WHERE act_id=$act_id AND emp_id=$employee->{emp_id}

Notice how this is different from 

  $schema->table('Activity')->fetch($act_id)

which would generate

  SELECT * FROM activity WHERE act_id=$act_id

Both examples would end up with the same record, but in the first
case there is an additional check that this record really
belongs to the given employee.

In presence of C<-fetch>, arguments C<-where> and C<-select_as> are not
allowed.


=item C<< -group_by => "string" >>  or C<< -group_by => \@array >> 

adds a C<GROUP BY> clause in the SQL statement. Grouping columns are
specified either by a plain string or by an array of strings.

=item C<< -having => "string" >>  or C<< -having => \%criteria >> 

adds a C<HAVING> clause in the SQL statement (only makes
sense together with a C<GROUP BY> clause).
This is like a C<-where> clause, except that the criteria
are applied after grouping has occured.

=item C<< -order_by => \@order >> 

C<< \@order >> is a reference to a list 
of columns for sorting. It can also be a plain SQL string
like C<< "col1 DESC, col3, col2 DESC" >>. Columns can 
also be prefixed by '+' or '-' for indicating sorting directions,
so for example C<< -orderBy => [qw/-col1 +col2 -col3/] >>
will generate the SQL clause
C<< ORDER BY col1 DESC, col2 ASC, col3 DESC >>.


=item C<< -for => $clause >> 

specifies an additional clause to be added at the end of the SQL statement,
like C<< -for => 'read only' >> or C<< -for => 'update' >>.

=item C<< -post_SQL => sub{...} >>

hook for specifying a callback function to be called on SQL code and

bind values, before preparing the statement. It will be called as
follows:

  ($sql, @bind) = $args->{-post_SQL}->($sql, @bind) if $args->{-post_SQL};


=item C<< -pre_exec => sub{...}, -post_exec => sub{...} >>

hooks for specifying callback functions to be called on the DBI statement
handle, just before or just after invoking C<< execute() >>. So the sequence
will be more or less like this:

  $sth = $dbh->prepare($sql_statement);
  $pre_exec_callback->($sth)  if $pre_exec_callback;
  $sth->execute(@bind_values);
  $post_exec_callback->($sth) if $post_exec_callback;

This is mostly useful if you need to call driver-specific functions at 
those stages.

=item C<< -post_bless => sub{...} >>

hook for specifying a callback function to be called on data rows.
The callback will be called I<after> L<bless_from_DB|bless_from_DB>, 
i.e. the row is already an object of the proper class and 
column handlers have been applied.


=item C<< -prepare_attrs => \%attrs >>

Optional attributes that will be transmitted
to L<DBI/prepare>.

=item C<< -page_size => $page_size >>

specifies how many rows will be retrieved per "page" of data.
Default is unlimited (or more precisely the maximum 
value of a short integer on your system).
When specified, automatically implies C<< -limit >>.

=item C<< -page_index => $page_index >>

specifies the page number (starting at 1). Default is 1.
When specified, automatically implies C<< -offset >>.

=item C<< -limit => $limit >>

limit to the number of rows that will be retrieved.
Automatically implied by C<< -page_size >>.

=item C<< -offset => $offset >>

Automatically implied by C<< -page_index >>.

=item C<< -column_types => \%column_types >>

dynamically specifies some column types I<at the statement level>.
Usually column types are defined within tables at compile time, but
it may be necessary to defined additional types within the 
statement, for example when using database functions and/or aliases.
The argument is a hashref in which keys are names of column
types defined within the schema, and values are arrayrefs
of column names :

   select(-columns     => [qw/ MAX(date_col)|max_date 
                               MIN(date_col)|min_date ... /],
          -column_types => { Date => [qw/max_date min_date/] },
          ...)


=item C<< -dbi_prepare_method => $method_name >>

overrides the L</dbi_prepare_method()> specified at the schema level.


=item C<< -result_as => $result_kind >>

specifies what kind of result will be produced. 
The result is always a scalar and does not depend on the
calling context.
Possible result kinds are :

=over

=item B<rows>

The result will be a ref to an array of rows, blessed into objects of the 
class. This is the default result kind. If there are no data rows, a ref
to an empty list is returned.

=item B<firstrow>

The result will be just the first data row, blessed into an object of the 
class. If there is no data, C<undef> is returned.


=item B<hashref> || B<< [hashref => @cols] >>

The result will be a hashref. Keys in the hash correspond to distinct
values of the specified columns, and values are data row objects.
If the argument is given as C<< [hashref => @cols] >>, the column(s)
are specified by the caller; otherwise if the argument is given
as a simple string, C<@cols> will default to C<< $source->primary_key >>.
If there is more than one column, the result will be a tree of nested hashes.
This C<-result_as> is normally used only where the key fields values 
for each row are unique. If multiple rows are returned with the same
values for the key fields then later rows overwrite earlier ones.


=item B<flat_arrayref>

The result will be a ref to an array that concatenates results
from each row. Usually this is combined with a C<-columns>
argument with one single column, to get a vertical slice from a resultset,
like in 

  my $all_names = $schema->table('People')->select(
    -columns   => [-DISTINCT => qw/firstname/],
    -result_As => 'flat_arrayref',
   );
  print sort @$all_names;

However, it may also be used for example to fill a hash from
pairs retrieved from each row, like in 

  my $pairs = $schema->table('People')->select(
    -columns   => [qw/pers_id firstname/],
    -result_as => 'flat_arrayref'
   );
  my %hash = @$pairs;

Finally, it can be convenient for avoiding column aliases,
when using aggregator functions :

  my $array_ref = $source->select(-columns  => [qw/MAX(col1)
                                                  AVG(col2)
                                                  COUNT(DISTINCT(col3))/],
                                  -where     => ...,
                                  -result_as => 'flat_arrayref');
  my ($max_col1, $avg_col2, $count_col3) = @$array_ref;


=item B<statement>

Returns the statement itself; data rows will be retrieved
later, through the L</next()> or L</all()> methods.
A typical usage pattern is :

  my $statement = $schema->table($table_name)->select(
    -where     => \%criteria, 
    -result_as => 'statement',
  );
  while (my $row = $statement->next) {
    do_something_with($row);
  }


=item B<fast_statement>

The result is like a normal statement, except that rows will be successively
fetched into the same memory location, using DBI's L<fetch|DBI/fetch>
and L<bind_columns|DBI/bind_columns> methods. This is the fastest way
to get data; however, pay attention to the following warning from L<DBI's
documentation|DBI/fetchrow_arrayref> : I<Note that the same array
reference is returned for each fetch, so don't store the reference and
then use it after a later fetch. Also, the elements of the array are
also reused for each row, so take care if you want to take a reference
to an element>.

Since each row must be retrieved separately,
call C<< all() >> or C<< next(10) >> on a fast statement
make no sense and therefore generate an exception. 

=item B<sth> 

The result will be an executed C<DBI> statement handle. Then it is up to the 
caller to retrieve data rows using the DBI API.
If needed, these rows can be later blessed into appropriate objects
through L<bless_from_DB()|/"bless_from_DB">.


=item B<sql> 

In scalar context, the result will just be the generated SQL statement. 
In list context, it will be C<($sql, @bind)>, i.e. the SQL statement 
together with the bind values.

=item B<subquery> 

Returns a ref to an arrayref containing C<< \["($sql)", @bind] >>. 
This is meant to be passed to a second
query through L<SQL::Abstract|SQL::Abstract>, as in :

  my $subquery = $source1->select(..., -result_as => 'subquery');
  my $rows     = $source2->select(
      -columns => ...,
      -where   => {foo => 123, bar => {-not_in => $subquery}}
   );

=back

=back



=head2 Retrieving data rows

=head3 next()

  while (my $row = $statement->next) {...}

  my $slice_arrayref = $statement->next(10);

If called without argument, returns the next data row, or C<undef> if
there are no more data rows.  If called with a numeric argument,
attempts to retrieve that number of rows, and returns an arrayref; the
size of the array may be smaller than required, if there were no more
data rows in the database. The numeric argument is forbidden when
L<select()|/select()> was called with C<< -result_as => 'fast_statement' >>,
because in that case rows must be retrieved one at a time.

Each row is blessed into an object of the proper class,
and is passed to the C<-post_bless> callback (if applicable).

=head3 all()

  my $rows = $statement->all;

Similar to the C<next> method, but 
returns an arrayref containing all remaining rows.
This method is forbidden when L<select()|/select()> was called with 
C<< -result_as => 'fast_statement' >>,
because in that case rows must be retrieved one at a time.


=head3 row_count()

Returns the number of rows corresponding to the current
executed statement. Raises an exception if the statement
is not in state C<"executed">.

The default implementation for counting rows involves an additional
call to the database (C<SELECT COUNT(*) FROM ...>); but a C<Statement>
subclass may override this method if the database driver provides a
better way (for example L<DBIx::DataModel::Statement::JDBC> calls the 
Java method C<getMemberCount>).



=head3 row_num()

Returns the index number of the next row to be fetched
(starting at C<< $self->offset >>, or 0 by default).



=head2 Pagination

=head3 page_size()

Returns the page size (requested number of rows), as it was set 
through the C<-page_size> argument to C<refine()> or C<select()>.

=head3 page_index()

Returns the current page index (starting at 1).
Always returns 1 if no pagination is activated
(no C<-page_size> argument was provided).

=head3 offset()

Returns the current I<requested> row offset (starting at 0).
This offset changes when a request is made to go to another page;
but it does not change when retrieving successive rows through the 
L</next> method.

=head3 page_count()

Calls L</row_count()> to get the total number of rows
for the current statement, and then computes the
total number of pages.

=head3 goto_page()

  $statement->goto_page($page_index);

Goes to the beginning of the specified page; usually this
involves a new call to L</execute>, unless the current
statement has methods to scroll through the result set
(see for example L<DBIx::DataModel::Statement::JDBC>).

Like for Perl arrays, a negative index is interpreted
as going backwards from the last page.

=head3 shift_pages()

  $statement->shift_pages($delta);

Goes to the beginning of the page corresponding to
the current page index + C<$delta>.

=head3 next_page()

A shortcut for C<< $statement->shift_pages(1) >>.

=head3 page_boundaries()

  my ($first, $last) = $statement->page_boundaries;

Returns the indices of first and last rows on the current page.
These numbers are given in "user coordinates", i.e. starting
at 1, not 0 : so if C<-page_size> is 10 and C<-page_index> is 
3, the boundaries are 21 / 30, while technically the current
offset is 20. On the last page, the C<$last> index corresponds
to C<row_count> (so C<$last - $first> is not always equal
to C<pageSize + 1>).

=head3 page_rows()

Returns an arrayref of rows corresponding to the current page
(maximum C<-page_size> rows).

=head2 Dealing with the statement lifecycle

=head3 status()

Returns the current status or the statement. This is a
L<dualvar|Scalar::Util/dualvar> with a
string component (C<new>, C<sqlized>, C<prepared>, C<executed>)
and an integer component (1, 2, 3, 4).

=head3 refine()

  $statement->refine(%args);

Set up some named parameters on the statement, that
will be used later when calling L</sqlize()>, L</prepare()>
and L</execute()>. Admitted parameters are listed under
the L</select()> method.

For most parameters, when C<refine()> is called several times on the
same statement, the latest value takes precedence (like the latest
update in a Perl hash) : this is the case for instance for parameters
C<-columns>, C<-order_by>, etc. However, for the C<-where> parameter,
the statement accumulates all successive conditions, combining them
with an implicit "AND". So for example

   $statement->refine(-where => {col1 => $val1, col2 => {">" => $min}})
             ->refine(-where => {col3 => $val3, col2 => {"<" => $max}});

is equivalent to

   $statement->refine(-where => {col1 => $val1, 
                                 col2 => {">" => $min,
                                          "<" => $max},
                                 col3 => $val3});

Refining a statement can only occur when the statement is still
in status C<NEW>; after L</sqlize()> has been called, 
parameters are frozen.

=head3 sqlize()

  $statement->sqlize(%args);

Generates SQL from all parameters accumulated so far in the statement.
The statement switches from state C<NEW> to state C<SQLIZED>,
which forbids any further refinement of the statement
(but does not forbid further bindings).

Arguments are optional, and are just a shortcut instead of writing

  $statement->refine(%args)->sqlize;

=head3 prepare()

  $statement->prepare(%args);

Method C<sqlized> is called automatically if necessary.
Then the SQL is sent to the database, and the statement handle
returned by DBI (C<$sth>)
is stored internally within the statement.
The state switches to C<PREPARED>.

Arguments are optional, and are just a shortcut instead of writing

  $statement->sqlize(%args)->prepare;

=head3 execute()

  $statement->execute(@bindings);

Translates the internal named bindings into positional
bindings, calls L<DBI/execute> on the internal C<$sth>, 
and applies the C<-pre_exec> and C<-post_exec> callbacks 
if necessary.
The state switches to C<EXECUTED>.

Arguments are optional, and are just a shortcut instead of writing

  $statement->bind(@bindings)->execute;

An executed statement can be executed again, possibly with some 
different bindings. When this happens, the internal result set
is reset, and fresh data rows can be retrieved again through 
the L</next> or L</all> methods.


=head3 sql()

  $sql         = $statement->sql;
  (sql, @bind) = $statement->sql;

In scalar context, returns the SQL code for this
statement (or C<undef> if the statement is not
yet C<sqlized>). 

In list context, returns the SQL code followed
by the bind values, suitable for a call to 
L<DBI/execute>.

Obviously, this method is only available after the
statement has been sqlized (through direct call 
to the L</sqlize> method, or indirect call via
L</prepare>, L</execute> or L</select>).

=head3 bind()

  $statement->bind(foo => 123, bar => 456);
  $statement->bind({foo => 123, bar => 456}); # equivalent to above

  $statement->bind(0 => 123, 1 => 456);
  $statement->bind([123, 456]);               # equivalent to above

Takes a list of bindings (name-value pairs), and associates
them to I<named placeholders> within the statement. Named
placeholders are defined at the C<DBIx::DataModel> level, not
within the database.

If successive bindings occur on the same named placeholder, the last
value silently overrides previous values. If a binding has no
corresponding named placeholder, it is ignored.  Names can be any
string (including numbers), except reserved words C<limit> and
C<offset>, which have a special use for pagination.

The list may alternatively be given as a hashref. This 
is convenient for example in situations like

  my $rows      = $source->select(...);   
  my $statement = $source->some_method;
  foreach my $row (@$rows) {
    my $related_rows = $statement->bind($row)->select;
  }

The list may also be given as an
arrayref; this is equivalent to a hashref
in which keys are positions within the array.

Finally, there is a ternary form 
of C<bind> for passing DBI-specific arguments.

  use DBI qw/:sql_types/;
  $statement->bind(foo => $val, {TYPE => SQL_INTEGER});

See L<DBI/"bind_param"> for explanations.


=head3 reset()

  $statement->reset(%args);

Resets the statement back into C<NEW> state, erasing all information except references
to the C<$schema> and C<$meta_source>.

C<%args> are optional; if present, they are passed to the L</refine()> method.



=head2 Proxy methods

Since the statement holds references to a schema and a meta-source,
there are some accessors or proxy methods to these objects.

=head3 schema()

Accessor for the statement's schema.

=head3 source()

Accessor for the statement's meta-source.


=head2 Utility methods

=head3 bless_from_DB()

  $statement->bless_from_DB($record);

Blesses C<< $record >> into an object of 
C<< $statement->meta_source->class >>,
and applies the C<from_DB> column handlers.


=head1 SOURCES

=head2 Principle

Data rows coming from the database are blessed into I<source classes>,
which are either I<tables> or I<joins>; such classes implement
instance methods for manipulating the row objects, or for navigating
to related rows. Source classes do not contain information about the
data structure (which database table, which associations, etc.); for
getting such information, use the C<metadm> method to access the
I<metasource> object.


=head2 Path methods specific to each source

When an L</Association()> is defined between two tables, 
methods are automatically added into the corresponding classes;
so for example with 

  $schema->Association([qw/Department department 1 /],
                       [qw/Activity   activities * /]);

the C<Department> class will have an C<activities()> method, and
the C<Activity> class will have a C<department()> method.
Such methods are called I<path methods>; their
names correspond to the UML roles defined in the association.
Path methods always take the same arguments as the L</select()> method
(actually, they are implemented through an internal call to C<select()>).

UML role names should preferably be chosen to avoid collisions
with the builtin methods listed below. However, should a conflict
occur, it is always possible to alleviate the ambiguity using
a fully qualified method name: for instance if a table has a
path method that collides with the L</schema()> method presented below, 
we can write

  # calling the path method
  my $related_row_called_schema = $data_row->schema(); 

  # calling the builtin method
  my $dbidm_schema = $data_row->DBIx::DataModel::Source::schema();

A join between several tables (see the L</join()> method) creates
a new class that inherits all path methods from all tables participating
in the join; in case of name conflics, the latest table takes precedence.
Again, it is possible to use fully qualified method names if necessary.


=head2 Accessors

=head3 Source::metadm()

Returns the meta-source instance associated with the invocant.


=head3 schema()

Returns the instance of L<DBIx::DataModel::Schema> from which
the current data row was retrieved. When in single-schema mode,
the schema comes from C<< $self->metadm->schema->class->singleton >>; 
when in multi-schema mode, a reference to the schema 
is kept under C<__schema> within each row object.


=head3 primary_key()

  my @primary_key_columns = $class->primary_key;
  my @primary_key_values  = $object->primary_key;

If called as a class method, returns the list of columns
registered as primary key for that table (via L</Table()>),
or computed as primary key for that view (concatenation of primary
keys of joined tables that are in a 1-to-many association).

If called as an instance method, returns the list of values 
in those columns.

When called in scalar context and the primary key has only one column,
returns that column (so you can call C<< my $k = $obj->primary_key >>).


=head2 Data manipulation

=head3 expand()

  $row->expand($path, %options);

Executes the method C<< $role >> to follow an Association,
stores the result in the object itself under C<< $row->{$path} >>,
and returns that result.
This is typically used to expand an object into a tree datastructure.
If present, C<< %options >> are passed to C<< $row->$path(%options) >>, for
example for specifying C<-where>, C<-columns> or C<-order_by> options.

After the expansion, further calls to 
C<< $row->$path >> (without any arguments) will reuse 
that same expanded result instead of calling the database again.
This caching improves efficiency, but also introduces the risk
of side-effects across your code : after 

  $row->expand(some_path => (-columns => [qw/just some columns/],
                             -where   => {some_field => 'restriction'}))

further calls to C<< $row->some_path() >> will just return
a dataset restricted according to the above criteria, instead
of a full join. To prevent that effect, you would need to 
C<< delete $row->{some_path} >>, or to call the role
with arguments, like C<< $row->some_path(-columns => '*') >>.



=head3 auto_expand()

  $record->auto_expand( $with_recursion );

Asks the object to expand itself with some objects in foreign tables.
Does nothing by default. Should be redefined in subclasses,
most probably through the 
L</set_auto_expand> method.
If the optional argument C<$with_recursion> is true, then 
C<auto_expand> is recursively called on the expanded objects.



=head2 apply_column_handler()

  $class ->apply_column_handler($handler_name, \@rows);
  $row   ->apply_column_handler($handler_name);

Inspects the target object or list of objects; for every
column that exists in the object, checks whether
a handler named C<< $handler_name >> was declared for
that column (see method L</set_column_handlers>), and if so, 
calls the handler. By this definition, if a column
is I<absent> in an object, then the handler for that column
is not called, even though it was declared in the class.

The results of handler calls are collected into a hashref, with an
entry for each column name.  The value of each entry depends on how
C<< apply_column_handlers >> was called : if it was called as an
instance method, then the result is something of shape

  {column_name1 => result_value1, column_name2 => result_value2, ... }

if it was called as a class method (i.e. if C<< \@objects >> is defined),
then the result is something of shape

  {column_name1 => [result_value1_for_object1, result_value1_for_object2, ...], 
   column_name2 => [result_value2_for_object1, result_value2_for_object2, ...], 
   ... }

If C<column_name> is not present in the target object(s), then the 
corresponding result value is  C<undef>.


=head2 has_invalid_columns

  my $invalid_columns = $row->has_invalid_columns;
  if ($invalid_columns) {
    print "wrong data in columns ", join(", ", @$invalid_columns);
  }
  else {
   print "all columns OK";
  }


Applies the 'validate' handler to all existent columns.
Returns a ref to the list of invalid columns, or
C<undef> if there are none.

Note that this is validation at the column level, not at the record
level. As a result, your validation handlers can check if an existent
column is empty, but cannot check if a column is missing (because in
that case the handler would not be called).

Your 'validate' handlers, defined through L</set_column_handlers>,
should return 0 or an empty string whenever the column value is invalid.
Never return C<undef>, because we would no longer be able to
distinguish between an invalid existent column and a missing column.


=head2 Methods delegated to the ConnectedSource class

=head3 Table::update()

  $source_class->update(@update_args);
  $source_instance->update({field1 => $val1, ...});
  $source_instance->update();


This is both a class and an instance method, that 
implicitly creates a a connected source from  the source's schema
and meta-source, and then calls the C<update()> method on that 
connected source. 

When used as a class method, the argument syntax is exactly like
for L<Connected_source::update()|update>.

When used as an instance method with arguments, this is equivalent to 

  $connected_source->update($source_instance->primary_key,
                            {field1 => $val1, ...});

in other words, the invocant is only used for taking
its primary key, and the updated fields are passed
as an argument hashref.


When used as an instance method without arguments, this is equivalent to

  $connected_source->update($source_instance);

in other words, the updated fields are all column values stored 
in memory within the object (ignoring all non-scalar values). 


=head2 Table::delete()

  $source_class->delete(@delete_args);
  $source_instance->delete();

Exactly like the C<update()> method just above, but
calling C<ConnectedSource::delete()>.


=head2 Table::join()


Calls L</"ConnectedSource::join()"> to create a statement, and
immediately binds it to the current object. So for example if 
C<< $emp->{emp_id} == 987 >>, then

  $emp->join(qw/activities department/)
      ->select(-where => {d_end => undef})

will generate

  SELECT * FROM Activity INNER JOIN Department 
                         ON Activity.dpt_id = Department.dpt_id
           WHERE emp_id = 987 
             AND d_end IS NULL

Observe that the C<WHERE> clause contains a combination of
criteria, coming on one hand from the initial C<$emp> object, 
and on the other hand from the regular C<-where> clause within the 
C<select()>.



=head2 class methods delegated to the ConnectedSource class

When in single-schema mode, some class methods
are delegated to a connecte source created on the fly, 
using the L</singleton()> method; hence it is possible to write requests 
like

  my $result = HR::Employee->select(...);

instead of 

  my $result = HR->table('Employee')->select(...);


=head3 Source::select()

  my $result = $source->select(%args);

Equivalent to 

  my $metadm      = $source->metadm;
  my $meta_schema = $metadm->schema;
  my $schema      = $meta_schema->class->singleton;
  my $cs_class    = $meta_schema->connected_source_class;
  $cs_class->new($metadm, $schema)->select(%args);

=head3 Source::fetch()

similar to C<select()> above

=head3 Source::fetch_cached()

similar to C<select()> above

=head3 Source::bless_from_DB()

similar to C<select()> above


=head1 META-SCHEMA NAVIGATION

=head2 Meta-schema methods

=head3 tables()

  my @meta_tables  = $meta_schema->tables;

Returns all L<DBIx::DataModel::Meta::Source::Table> instances
declared in this C<$meta_schema>.

=head3 Meta::Schema::table()

  my $meta_table   = $meta_schema->table($table_name);

Returns the single instance of L<DBIx::DataModel::Meta::Source::Table> 
with name C<$table_name>, or C<undef>.

=head3 associations()

  my @associations = $meta_schema->associations;

Returns all L<DBIx::DataModel::Meta::Association> instances
declared in this C<$meta_schema>.

=head3 association()

  my $association  = $meta_schema->associations($association_name);

Returns the single instance of L<DBIx::DataModel::Meta::Source::Association> 
with name C<$association_name>, or C<undef>.


=head3 types()

  my @types        = $meta_schema->types;

Returns all L<DBIx::DataModel::Meta::Type> instances
declared in this C<$meta_schema>.

=head3 type()

  my $type         = $meta_schema->type($type_name);

Returns the single instance of L<DBIx::DataModel::Meta::Type> 
with name C<$type_name>, or C<undef>.

=head3 joins()

  my @joins        = $meta_schema->joins;

Returns all L<DBIx::DataModel::Meta::Source::Join> instances
declared in this C<$meta_schema>.

=head3 Meta::Schema::join()

  my $join         = $meta_schema->join($join_name);

Returns the single instance of L<DBIx::DataModel::Meta::Source::Join> 
with name C<$join_name>, or C<undef>.
  
=head3 other accessors

Accessor methods are defined for the following members of the
C<$meta_schema> : 

=over

=item class()

=item sql_no_inner_after_left_join()

=item auto_insert_columns()

=item auto_update_columns()

=item no_update_columns()

=item table_parent()

=item table_metaclass()

=item join_parent()

=item join_metaclass()

=item association_metaclass()

=item path_metaclass()

=item type_metaclass()

=item statement_class()

=item connected_source_class()

=back


=head2 Meta-source methods

Accessor methods are defined for the following members of a 
C<$meta_source> (instance of either
L<DBIx::DataModel::Meta::Source::Table> or 
L<DBIx::DataModel::Meta::Source::Join>) : 

=over

=item schema()

=item class()

=item default_columns()

=item parents()

=item primary_key()

=item aliased tables()

=item name()

=back

In addition, the following methods return dynamic lists :

=head3 ancestors()

Returns a flattened list of recursive calls to the 
L</parents()> method.

=head3 auto_insert_column()

Returns a flattened hash, built from C<auto_insert_columns> declared
in this source, in its ancestors, or in the C<$meta_schema>.

=head3 auto_update_column()

Returns a flattened hash, built from C<auto_update_columns> declared
in this source, in its ancestors, or in the C<$meta_schema>.

=head3 no_update_column()

Returns a flattened hash, built from C<no_update_columns> declared
in this source, in its ancestors, or in the C<$meta_schema>.
Keys are column names, values are insignificant.

=head3 path()

  my %all_path      = $meta_source->path;
  my $specific_path = $meta_source->path($path_name);

Returns a flattened hash, built from C<no_update_columns> declared
in this source, in its ancestors, or in the C<$meta_schema>.
If a C<$path_name>, returns the corresponding entry in 
C<%all_path>.

=head3 db_from()

Returns what will be injected as
C<-from> argument into the 
L<SQL::Abstract::More/select> call.


=head3 where()

Returns the optional "where" condition associated with
this source (in the case of L</View()>).


=head2 Meta-table methods

In addition to the C<$meta_source> methods above, 
the following methods are defined for an instance
of L<DBIx::DataModel::Meta::Source::Table> :


=head3 components()

Returns the list of other meta-sources that have 
been declared as components of this source, through 
the L</Composition()> declaration.


=head2 Association methods

=head3 schema()

The L<DBIx::DataModel::Meta::Schema> instance in which
this association is declared.


=head3 name()

Returns the association name

=head3 kind()

Returns the association kind (C<Association> or C<Composition>).

=head3 path_AB()

Returns the L<DBIx::DataModel::Meta::Path> object describing
the path from A to B.


=head3 path_BA()

Returns the L<DBIx::DataModel::Meta::Path> object describing
the path from B to A.


=head2 Path methods

=head3 name()

The name of this path.

=head3 from()

Reference to the  L<DBIx::DataModel::Meta::Source::Table> where this
path starts.

=head3 to()

Reference to the  L<DBIx::DataModel::Meta::Source::Table> where this
path ends.

=head3 on()

Hashref for generating the join condition (keys are colums for the left-hand
side, values are columns for the right-hand side).

=head3 multiplicity()

see L</Association methods>

=head3 association()

Reference to the  L<DBIx::DataModel::Meta::Association> that created
this path.

=head3 direction()

Either C<"AB"> or C<"BA">.

=head3 opposite()

Returns the path object representing the opposite direction.

=head2 Type methods

=head3 schema()

The L<DBIx::DataModel::Meta::Schema> instance in which
this type is declared.

=head3 name()

Name of this type.

=head3 handlers()

Hashref of handlers declared in this type (keys are handler names, values are
handler bodies, i.e. coderefs).


