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[dbsrgits/DBIx-Class.git] / lib / DBIx / Class / Relationship.pm

package DBIx::Class::Relationship;

use strict;
use warnings;

use base qw/DBIx::Class/;

__PACKAGE__->load_own_components(qw/
  Helpers
  Accessor
  CascadeActions
  ProxyMethods
  Base
/);

=head1 NAME 

DBIx::Class::Relationship - Inter-table relationships

=head1 SYNOPSIS

=head1 DESCRIPTION

This class provides methods to set up relationships between the tables
in your database model. Relationships are the most useful and powerful
technique that L<DBIx::Class> provides. To create efficient database queries,
create relationships between any and all tables that have something in
common, for example if you have a table Authors:

  ID  | Name | Age
 ------------------
   1  | Fred | 30
   2  | Joe  | 32

and a table Books:

  ID  | Author | Name
 --------------------
   1  |      1 | Rulers of the universe
   2  |      1 | Rulers of the galaxy

Then without relationships, the method of getting all books by Fred goes like
this:

 my $fred = $schema->resultset('Author')->find({ Name => 'Fred' });
 my $fredsbooks = $schema->resultset('Book')->search({ Author => $fred->ID });
With a has_many relationship called "books" on Author (see below for details),
we can do this instead:

 my $fredsbooks = $schema->resultset('Author')->find({ Name => 'Fred' })->books;

Each relationship sets up an accessor method on the 
L<DBIx::Class::Manual::Glossary/"Row"> objects that represent the items
of your table. From L<DBIx::Class::Manual::Glossary/"ResultSet"> objects,
the relationships can be searched using the "search_related" method. 
In list context, each returns a list of Row objects for the related class,
in scalar context, a new ResultSet representing the joined tables is
returned. Thus, the calls can be chained to produce complex queries.
Since the database is not actually queried until you attempt to retrieve
the data for an actual item, no time is wasted producing them.

 my $cheapfredbooks = $schema->resultset('Author')->find({ Name => 'Fred' })->books->search_related('prices', { Price => { '<=' => '5.00' } });

will produce a query something like:

 SELECT * FROM Author me 
 LEFT JOIN Books books ON books.author = me.id
 LEFT JOIN Prices prices ON prices.book = books.id
 WHERE prices.Price <= 5.00

all without needing multiple fetches.

Only the helper methods for setting up standard relationship types
are documented here. For the basic, lower-level methods, see
L<DBIx::Class::Relationship::Base>.

=head1 METHODS

All helper methods take the following arguments:

  __PACKAGE__>$method_name('relname', 'Foreign::Class', $cond, $attrs);
  
Both C<$cond> and C<$attrs> are optional. Pass C<undef> for C<$cond> if
you want to use the default value for it, but still want to set C<$attrs>.
See L<DBIx::Class::Relationship::Base> for a list of valid attributes.

=head2 belongs_to

  # in a Book class (where Author has many Books)
  My::DBIC::Schema::Book->belongs_to(author => 'Author');
  my $author_obj = $obj->author;
  $obj->author($new_author_obj);

Creates a relationship where the calling class stores the foreign class's 
primary key in one (or more) of its columns. If $cond is a column name
instead of a join condition hash, that is used as the name of the column
holding the foreign key. If $cond is not given, the relname is used as
the column name.

NOTE: If you are used to L<Class::DBI> relationships, this is the equivalent
of C<has_a>.

=head2 has_many

  # in an Author class (where Author has many Books)
  My::DBIC::Schema::Author->has_many(books => 'Book', 'author');
  my $booklist = $obj->books;
  my $booklist = $obj->books({ name => { LIKE => '%macaroni%' }, { prefetch => [qw/book/] });
  my @book_objs = $obj->books;

  $obj->add_to_books(\%col_data);

Creates a one-to-many relationship, where the corresponding elements of the
foreign class store the calling class's primary key in one (or more) of its
columns. You should pass the name of the column in the foreign class as the
$cond argument, or specify a complete join condition.

If you delete an object in a class with a C<has_many> relationship, all
related objects will be deleted as well. However, any database-level
cascade or restrict will take precedence.

=head2 might_have

  My::DBIC::Schema::Author->might_have(psuedonym => 'Psuedonyms');
  my $pname = $obj->psuedonym; # to get the Psuedonym object

Creates an optional one-to-one relationship with a class, where the foreign
class stores our primary key in one of its columns. Defaults to the primary
key of the foreign class unless $cond specifies a column or join condition.

If you update or delete an object in a class with a C<might_have>
relationship, the related object will be updated or deleted as well.
Any database-level update or delete constraints will override this behaviour.

=head2 has_one

  My::DBIC::Schema::Book->has_one(isbn => ISBN);
  my $isbn_obj = $obj->isbn;

Creates a one-to-one relationship with another class. This is just like
C<might_have>, except the implication is that the other object is always
present. The only difference between C<has_one> and C<might_have> is that
C<has_one> uses an (ordinary) inner join, whereas C<might_have> uses a
left join.


=head2 many_to_many

  My::DBIC::Schema::Actor->many_to_many( roles => 'actor_roles', 'Roles' );
  my @role_objs = $obj_a->roles;

Creates an accessor bridging two relationships; not strictly a relationship
in its own right, although the accessor will return a resultset or collection
of objects just as a has_many would.

=cut

1;

=head1 AUTHORS

Matt S. Trout <mst@shadowcatsystems.co.uk>

=head1 LICENSE

You may distribute this code under the same terms as Perl itself.

=cut