Update docs for best practices and correctness.

[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

  ## Creating relationships
  MyDB::Schema::Actor->has_many('actorroles' => 'MyDB::Schema::ActorRole',
                                'actor');
  MyDB::Schema::Role->has_many('actorroles' => 'MyDB::Schema::ActorRole',
                                'role');
  MyDB::Schema::ActorRole->belongs_to('role' => 'MyDB::Schema::Role');
  MyDB::Schema::ActorRole->belongs_to('actor' => 'MyDB::Schema::Actor');

  MyDB::Schema::Role->many_to_many('actors' => 'actorroles', 'actor');
  MyDB::Schema::Actor->many_to_many('roles' => 'actorroles', 'role');

  ## Using relationships
  $schema->resultset('Actor')->roles();
  $schema->resultset('Role')->search_related('actors', { Name => 'Fred' });
  $schema->resultset('ActorRole')->add_to_roles({ Name => 'Sherlock Holmes'});

See L<DBIx::Class::Manual::Cookbook> for more.

=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, and a description
of all the useful *_related methods that you get for free, see
L<DBIx::Class::Relationship::Base>.

=head1 METHODS

All helper methods are called similar to the following template:

  __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 documentation on the
attrubutes that are allowed in the C<$attrs> argument.


=head2 belongs_to

=over 4

=item Arguments: $accessor_name, $related_class, $fk_column|\%cond|\@cond?, \%attr?

=back

Creates a relationship where the calling class stores the foreign
class's primary key in one (or more) of its columns. This relationship
defaults to using C<$accessor_name> as the column in this class
to resolve the join against the primary key from C<$related_class>,
unless C<$fk_column> specifies the foreign key column in this class or
C<cond> specifies a reference to a join condition hash.

=over

=item accessor_name

This argument is the name of the method you can call on a
L<DBIx::Class::Row> object to retrieve the instance of the foreign
class matching this relationship. This is often called the
C<relation(ship) name>.

Use this accessor_name in L<DBIx::Class::ResultSet/join>
or L<DBIx::Class::ResultSet/prefetch> to join to the foreign table
indicated by this relationship.

=item related_class

This is the class name of the table referenced by the foreign key in
this class.

=item fk_column

The column name on this class that contains the foreign key.

OR

=item cond

A hashref where the keys are C<foreign.$column_on_related_table> and
the values are C<self.$foreign_key_column>. This is useful for
relations that are across multiple columns.

=back


  # in a Book class (where Author has many Books)
  My::DBIC::Schema::Book->belongs_to( 
    author => 
    'My::DBIC::Schema::Author', 
    'author_id'
  );

  # OR (same result)
  My::DBIC::Schema::Book->belongs_to(
    author =>
    'My::DBIC::Schema::Author',
    { 'foreign.author_id' => 'self.author_id' } 
  );

  # OR (similar result but uglier accessor name)
  My::DBIC::Schema::Book->belongs_to( 
    author_id =>
    'My::DBIC::Schema::Author'
  );

  # Usage
  my $author_obj = $book->author; # get author object
  $book->author( $new_author_obj ); # set author object
  $book->author_id(); # get the plain id

  # To retrieve the plain id if you used the ugly version:
  $book->get_column('author_id');


If the relationship is optional -- i.e. the column containing the foreign key
can be NULL -- then the belongs_to relationship does the right thing. Thus, in
the example above C<$obj-E<gt>author> would return C<undef>.  However in this
case you would probably want to set the C<join_type> attribute so that a C<LEFT
JOIN> is done, which makes complex resultsets involving C<join> or C<prefetch>
operations work correctly.  The modified declaration is shown below:

  # in a Book class (where Author has_many Books)
  __PACKAGE__->belongs_to(
    author => 
    'My::DBIC::Schema::Author',
    'author', 
    { join_type => 'left' }
  );


Cascading deletes are off by default on a C<belongs_to>
relationship. To turn them on, pass C<< cascade_delete => 1 >>
in the $attr hashref.

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

See L<DBIx::Class::Relationship::Base> for documentation on relationship
methods and valid relationship attributes.

=head2 has_many

=over 4

=item Arguments: $accessor_name, $related_class, $foreign_key_column|\%cond|\@cond?, \%attr?

=back

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. This relationship defaults to using C<$accessor_name> as the foreign
key in C<$related_class> to resolve the join, unless C<$foreign_key_column>
specifies the foreign key column in C<$related_class> or C<cond> specifies a
reference to a join condition hash.

=over

=item accessor_name

This argument is the name of the method you can call on a
L<DBIx::Class::Row> object to retrieve a resultset of the related
class restricted to the ones related to the row object. In list
context it returns the row objects. This is often called the
C<relation(ship) name>.

Use this accessor_name in L<DBIx::Class::ResultSet/join>
or L<DBIx::Class::ResultSet/prefetch> to join to the foreign table
indicated by this relationship.

=item related_class

This is the class name of the table which contains a foreign key
column containing PK values of this class.

=item foreign_key_column

The column name on the related class that contains the foreign key.

OR

=item cond

A hashref where the keys are C<foreign.$foreign_key_column> and
the values are C<self.$matching_column>. This is useful for
relations that are across multiple columns.

OR

An arrayref containing an SQL::Abstract-like condition. For example a
link table where two columns link back to the same table. This is an
OR condition.

  My::Schema::Item->has_many('rels', 'My::Schema::Relationships',
                             [ { 'foreign.LItemID' => 'self.ID' },
                               { 'foreign.RItemID' => 'self.ID'} ]);
=back

  # in an Author class (where Author has_many Books)
  My::DBIC::Schema::Author->has_many(
    books => 
    'My::DBIC::Schema::Book', 
    'author_id'
  );

  # OR (same result, assuming related_class is storing our PK)
  My::DBIC::Schema::Author->has_many(
    books => 
    'My::DBIC::Schema::Book', 
  );

  # OR (same result)
  My::DBIC::Schema::Author->has_many(
    books => 
    'My::DBIC::Schema::Book', 
    { 'foreign.author_id' => 'self.id' },
  );
  

  # Usage
  # resultset of Books belonging to author 
  my $booklist = $author->books;

  # resultset of Books belonging to author, restricted by author name
  my $booklist = $author->books({
    name => { LIKE => '%macaroni%' },
    { prefetch => [qw/book/],
  });

  # array of Book objects belonging to author
  my @book_objs = $author->books;

  # force resultset even in list context
  my $books_rs = $author->books;
  ( $books_rs ) = $obj->books_rs;

  # create a new book for this author, the relation fields are auto-filled
  $author->create_related('books', \%col_data);
  # alternative method for the above
  $author->add_to_books(\%col_data);


Three methods are created when you create a has_many relationship.  The first
method is the expected accessor method, C<$accessor_name()>.  The second is
almost exactly the same as the accessor method but "_rs" is added to the end of
the method name.  This method works just like the normal accessor, except that
it always returns a resultset, even in list context. The third method,
named C<< add_to_$relname >>, will also be added to your Row items; this
allows you to insert new related items, using the same mechanism as in
L<DBIx::Class::Relationship::Base/"create_related">.

If you delete an object in a class with a C<has_many> relationship, all
the related objects will be deleted as well.  To turn this behaviour off,
pass C<< cascade_delete => 0 >> in the C<attr> hashref. However, any
database-level cascade or restrict will take precedence over a
DBIx-Class-based cascading delete.

If you copy an object in a class with a C<has_many> relationship, all
the related objects will be copied as well. To turn this behaviour off,
pass C<< cascade_copy => 0 >> in the C<$attr> hashref. The behaviour
defaults to C<< cascade_copy => 1 >>.

See L<DBIx::Class::Relationship::Base> for documentation on relationship
methods and valid relationship attributes.

=head2 might_have

=over 4

=item Arguments: $accessor_name, $related_class, $foreign_key_column|\%cond|\@cond?, \%attr?

=back

Creates an optional one-to-one relationship with a class. This relationship
defaults to using C<$accessor_name> as the foreign key in C<$related_class> to
resolve the join, unless C<$foreign_key_column> specifies the foreign key
column in C<$related_class> or C<cond> specifies a reference to a join
condition hash.

=over

=item accessor_name

This argument is the name of the method you can call on a
L<DBIx::Class::Row> object to retrieve the instance of the foreign
class matching this relationship. This is often called the
C<relation(ship) name>.

Use this accessor_name in L<DBIx::Class::ResultSet/join>
or L<DBIx::Class::ResultSet/prefetch> to join to the foreign table
indicated by this relationship.

=item related_class

This is the class name of the table which contains a foreign key
column containing PK values of this class.

=item foreign_key_column

The column name on the related class that contains the foreign key.

OR

=item cond

A hashref where the keys are C<foreign.$column_on_related_table> and
the values are C<self.$foreign_key_column>. This is useful for
relations that are across multiple columns.

=back

  # Author may have an entry in the pseudonym table
  My::DBIC::Schema::Author->might_have(
    pseudonym =>
    'My::DBIC::Schema::Pseudonym',
    'author_id',
  );

  # OR (same result, assuming the related_class stores our PK)
  My::DBIC::Schema::Author->might_have(
    pseudonym =>
    'My::DBIC::Schema::Pseudonym',
  );

  # OR (same result)
  My::DBIC::Schema::Author->might_have(
    pseudonym =>
    'My::DBIC::Schema::Pseudonym',
    { 'foreign.author_id' => 'self.id' },
  );

  # Usage
  my $pname = $author->pseudonym; # to get the Pseudonym object

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. To
turn off this behavior, add C<< cascade_delete => 0 >> to the C<$attr>
hashref. Any database-level update or delete constraints will override
this behavior.

See L<DBIx::Class::Relationship::Base> for documentation on relationship
methods and valid relationship attributes.

=head2 has_one

=over 4

=item Arguments: $accessor_name, $related_class, $foreign_key_column|\%cond|\@cond?, \%attr?

=back

Creates a one-to-one relationship with a class. This relationship
defaults to using C<$accessor_name> as the foreign key in C<$related_class> to
resolve the join, unless C<$foreign_key_column> specifies the foreign key
column in C<$related_class> or C<cond> specifies a reference to a join
condition hash.

=over

=item accessor_name

This argument is the name of the method you can call on a
L<DBIx::Class::Row> object to retrieve the instance of the foreign
class matching this relationship. This is often called the
C<relation(ship) name>.

Use this accessor_name in L<DBIx::Class::ResultSet/join>
or L<DBIx::Class::ResultSet/prefetch> to join to the foreign table
indicated by this relationship.

=item related_class

This is the class name of the table which contains a foreign key
column containing PK values of this class.

=item foreign_key_column

The column name on the related class that contains the foreign key.

OR

=item cond

A hashref where the keys are C<foreign.$column_on_related_table> and
the values are C<self.$foreign_key_column>. This is useful for
relations that are across multiple columns.

=back

  # Every book has exactly one ISBN
  My::DBIC::Schema::Book->has_one(
    isbn => 
    'My::DBIC::Schema::ISBN',
    'book_id',
  );

  # OR (same result, assuming related_class stores our PK)
  My::DBIC::Schema::Book->has_one(
    isbn => 
    'My::DBIC::Schema::ISBN',
  );

  # OR (same result)
  My::DBIC::Schema::Book->has_one(
    isbn => 
    'My::DBIC::Schema::ISBN',
    { 'foreign.book_id' => 'self.id' },
  );

  # Usage
  my $isbn_obj = $book->isbn; # to get the ISBN object

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> defaults to a left join.

The has_one relationship should be used when a row in the table has exactly one
related row in another table. If the related row might not exist in the foreign
table, use the L<DBIx::Class::Relationship/might_have> relationship.

In the above example, each Book in the database is associated with exactly one
ISBN object.

See L<DBIx::Class::Relationship::Base> for documentation on relationship
methods and valid relationship attributes.

=head2 many_to_many

=over 4

=item Arguments: $accessor_name, $link_rel_name, $foreign_rel_name, \%attr?

=back

C<many_to_many> is not strictly a relationship in its own right. Instead, it is
a bridge between two resultsets which provide the same kind of convenience
accessors as true relationships provide. Although the accessor will return a 
resultset or collection of objects just like has_many does, you cannot call 
C<related_resultset> and similar methods which operate on true relationships.

=over

=item accessor_name

This argument is the name of the method you can call on a
L<DBIx::Class::Row> object to retrieve the rows matching this
relationship.

On a many_to_many, unlike other relationships, this cannot be used in
L<DBIx::Class::ResultSet/search> to join tables. Use the relations
bridged across instead.

=item link_rel_name

This is the accessor_name from the has_many relationship we are
bridging from.

=item foreign_rel_name

This is the accessor_name of the belongs_to relationship in the link
table that we are bridging across (which gives us the table we are
bridging to).

=back

To create a many_to_many relationship from Actor to Role:

  My::DBIC::Schema::Actor->has_many( actor_roles =>
                                     'My::DBIC::Schema::ActorRoles',
                                     'actor' );
  My::DBIC::Schema::ActorRoles->belongs_to( role =>
                                            'My::DBIC::Schema::Role' );
  My::DBIC::Schema::ActorRoles->belongs_to( actor =>
                                            'My::DBIC::Schema::Actor' );

  My::DBIC::Schema::Actor->many_to_many( roles => 'actor_roles',
                                         'role' );

And, for the reverse relationship, from Role to Actor:

  My::DBIC::Schema::Role->has_many( actor_roles =>
                                    'My::DBIC::Schema::ActorRoles',
                                    'role' );

  My::DBIC::Schema::Role->many_to_many( actors => 'actor_roles', 'actor' );

To add a role for your actor, and fill in the year of the role in the
actor_roles table:

  $actor->add_to_roles($role, { year => 1995 });

In the above example, ActorRoles is the link table class, and Role is the
foreign class. The C<$link_rel_name> parameter is the name of the accessor for
the has_many relationship from this table to the link table, and the
C<$foreign_rel_name> parameter is the accessor for the belongs_to relationship
from the link table to the foreign table.

To use many_to_many, existing relationships from the original table to the link
table, and from the link table to the end table must already exist, these
relation names are then used in the many_to_many call.

In the above example, the Actor class will have 3 many_to_many accessor methods
set: C<roles>, C<add_to_roles>, C<set_roles>, and similarly named accessors
will be created for the Role class for the C<actors> many_to_many
relationship.

See L<DBIx::Class::Relationship::Base> for documentation on relationship
methods and valid relationship attributes.

=cut

1;

=head1 AUTHORS

see L<DBIx::Class>

=head1 LICENSE

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

=cut