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Simon Elliott > DBIx-Class-0.06002 > DBIx::Class::Relationship



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


  MyDB::Schema::Actor->has_many('actorroles' => 'MyDB::Schema::ActorRole',
  MyDB::Schema::Role->has_many('actorroles' => 'MyDB::Schema::ActorRole',
  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');

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

See DBIx::Class::Manual::Cookbook for more.


This class provides methods to set up relationships between the tables in your database model. Relationships are the most useful and powerful technique that 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 "Row" in DBIx::Class::Manual::Glossary objects that represent the items of your table. From "ResultSet" in DBIx::Class::Manual::Glossary 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 =
 LEFT JOIN Prices prices ON =
 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 DBIx::Class::Relationship::Base.


All helper methods take the following arguments:

  __PACKAGE__>$method_name('relname', 'Foreign::Class', $cond, $attrs);

Both $cond and $attrs are optional. Pass undef for $cond if you want to use the default value for it, but still want to set $attrs. See DBIx::Class::Relationship::Base for a list of valid attributes.


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

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 Class::DBI relationships, this is the equivalent of has_a.


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


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.

As well as the accessor method, a method named 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 "create_related" in DBIx::Class::Relationship::Base.

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


  My::DBIC::Schema::Author->might_have(pseudonym =>
  my $pname = $obj->pseudonym; # to get the Pseudonym 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 might_have relationship, the related object will be updated or deleted as well. Any database-level update or delete constraints will override this behaviour.


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

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


  My::DBIC::Schema::Actor->has_many( actor_roles =>
                                     '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' );


  my @role_objs = $actor->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. 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.


Matt S. Trout <>


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

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