DBIx::C...tionship(3)

💅 DBIx::Class::Relationship — inter-table relationships, bestie

ok so this is the DBIx::Class::Relationship(3) page, straight out of the User Contributed Perl Documentation, and honestly? it's giving "the most powerful thing in the whole toolkit" energy. sit down, grab a strawberry drink 🍓, we're doing relationships.

✨ NAME ✨

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

✨ SYNOPSIS ✨

like, first you gotta create the relationships:

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

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

and then you get to use them, which is the fun part 💅:

## Using relationships
$schema->resultset('Actor')->find({ id => 1})->roles();
$schema->resultset('Role')->find({ id => 1 })->actorroles->search_related('actor', { Name => 'Fred' });
$schema->resultset('Actor')->add_to_roles({ Name => 'Sherlock Holmes'});

see DBIx::Class::Manual::Cookbook for more, bestie — there's a whole cookbook 🧚

✨ DESCRIPTION ✨

ok real talk — the word Relationship has a very specific meaning in DBIx::Class. check the definition in the Glossary if you want the official lore 👑

this class hands you the methods to set up relationships between the tables in your database model. and like… relationships are genuinely the most useful and most powerful technique DBIx::Class gives you. no cap. to create efficient database queries, you create relationships between any and all tables that have something in common. for example, if you've got 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, getting all of Fred's books is this whole ordeal 😭:

 my $fred = $schema->resultset('Author')->find({ Name => 'Fred' });
 my $fredsbooks = $schema->resultset('Book')->search({ Author => $fred->ID });

but with a has_many relationship called "books" on Author (details below, keep reading 🎀), we can do this instead:

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

slay. every relationship sets up an accessor method on the Result objects that represent the items of your table. and from ResultSet objects, you can search the relationships using the search_related method. in list context, each one returns a list of Result objects for the related class; in scalar context, you get a new ResultSet representing the joined tables. so the calls just chain and chain and produce complex queries like it's nothing. and because the database isn't actually queried until you try to retrieve the data for an actual item — no time is wasted producing them. iconic behavior honestly ✨

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

which 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. periodt. 🔥

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, go look at DBIx::Class::Relationship::Base. iykyk.

---

✨ METHODS ✨

all the helper methods get called kinda like this template:

  __PACKAGE__->$method_name('rel_name', 'Foreign::Class', \%cond|\@cond|\&cond?, \%attrs?);

both cond and attrs are optional 🫶. pass undef for cond if you want the default value for it but you still wanna set attrs. very "i want the default but also this one extra thing" of you.

see "condition" in DBIx::Class::Relationship::Base for the full documentation on how the cond argument is defined.

see "attributes" in DBIx::Class::Relationship::Base for the docs on which attributes are allowed in the attrs argument.

🎀 belongs_to

Arguments: $accessor_name, $related_class, $our_fk_column|\%cond|\@cond|\$cond?, \%attrs?

creates a relationship where the calling class stores the foreign class's primary key in one (or more) of the calling class's columns 💖. this relationship defaults to using $accessor_name as the column name in this class to resolve the join against the primary key from $related_class — unless $our_fk_column specifies the foreign key column in this class, or cond specifies a reference to a join condition.

accessor_name

this argument is the name of the method you can call on a Result object to retrieve the instance of the foreign class matching this relationship. people often call this the "relation(ship) name" 🎀.

use this accessor_name in "join" in DBIx::Class::ResultSet or "prefetch" in DBIx::Class::ResultSet to join to the foreign table indicated by this relationship.

related_class

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

our_fk_column

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

OR

cond

a hashref, arrayref or coderef specifying a custom join expression. for more info see "condition" in DBIx::Class::Relationship::Base.

  # 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');

ok so if some of the foreign key columns are nullable, you probably wanna set the join_type attribute to "left" explicitly so that the SQL expressing this relation is composed with a "LEFT JOIN" (as opposed to "INNER JOIN", which is the default for "belongs_to" relationships). this ensures relationship traversal works consistently in all situations (i.e. resultsets involving join or prefetch). 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 "belongs_to" relationship. to turn them on, pass "cascade_delete => 1" in the $attr hashref. (yes, that's the switch. be careful, bestie 💀)

by default, DBIC will return undef and avoid querying the database if a "belongs_to" accessor is called when any part of the foreign key IS NULL. to disable this behavior, pass "undef_on_null_fk => 0" in the "\%attrs" hashref.

NOTE: if you're used to Class::DBI relationships, this is the equivalent of "has_a". 🫶

see "attributes" in DBIx::Class::Relationship::Base for documentation on relationship methods and valid relationship attributes. also see DBIx::Class::ResultSet for a list of standard resultset attributes which can be assigned to relationships as well.

🌈 has_many

Arguments: $accessor_name, $related_class, $their_fk_column|\%cond|\@cond|\&cond?, \%attrs?

creates a one-to-many relationship where the foreign class refers to this class's primary key 🦋. this relationship refers to zero or more records in the foreign table (e.g. a "LEFT JOIN"). it defaults to using the end of this class's namespace as the foreign key in $related_class to resolve the join — unless $their_fk_column specifies the foreign key column in $related_class, or cond specifies a reference to a join condition.

accessor_name

this argument is the name of the method you can call on a Result object to retrieve a resultset of the related class restricted to the ones related to the result object. in list context it returns the result objects. people often call this the "relation(ship) name" ✨.

use this accessor_name in "join" in DBIx::Class::ResultSet or "prefetch" in DBIx::Class::ResultSet to join to the foreign table indicated by this relationship.

related_class

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

their_fk_column

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

OR

cond

a hashref, arrayref or coderef specifying a custom join expression. for more info see "condition" in DBIx::Class::Relationship::Base.

  # in an Author class (where Author has_many Books)
  # assuming related class is storing our PK in "author_id"
  My::DBIC::Schema::Author->has_many(
    books =>
    'My::DBIC::Schema::Book',
    'author_id'
  );

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

  # OR (similar result, assuming related_class is storing our PK, in "author")
  # (the "author" is guessed at from "Author" in the class namespace)
  My::DBIC::Schema::Author->has_many(
    books =>
    'My::DBIC::Schema::Book',
  );

  # 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 get created when you create a has_many relationship, and honestly they're a little trio 👑

1. the first method is the expected accessor method, $accessor_name().
2. the second is almost exactly the same as the accessor method, but "_rs" gets added to the end of the method name, eg $accessor_name_rs(). this one works just like the normal accessor, except it always returns a resultset — even in list context. 💅
3. the third method, named "add_to_$rel_name", also gets added to your Row items; this lets you insert new related items using the same mechanism as in "create_related" in DBIx::Class::Relationship::Base.

okay so deleting 🗑️ — if you delete an object in a class with a has_many relationship, all the related objects get deleted too, no survivors, that's just how it is. To turn this behaviour off, pass cascade_delete => 0 in the $attr hashref.

⚠️ real talk: the cascaded operations are performed after the requested delete or update. So if your database has a constraint on the relationship, it will have deleted/updated the related records or raised an exception before DBIx::Class gets to perform the cascaded operation. Order matters, bestie 💅

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

See "attributes" in DBIx::Class::Relationship::Base for the docs on relationship methods and valid relationship attributes. Also peep DBIx::Class::ResultSet for the list of standard resultset attributes you can assign to relationships as well.

---

🌸 might_have

Arguments: $accessor_name, $related_class, $their_fk_column|\%cond|\@cond|\&cond?, \%attrs?

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

accessor_name

This argument is the name of the method you can call on a Result object to retrieve the instance of the foreign class matching this relationship. This is often called the "relation(ship) name".

Use this accessor_name in "join" in DBIx::Class::ResultSet or "prefetch" in DBIx::Class::ResultSet to join to the foreign table indicated by this relationship 🔗

related_class

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

their_fk_column

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

OR

cond

A hashref, arrayref or coderef specifying a custom join expression. For more info see "condition" in DBIx::Class::Relationship::Base.

  # 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 might_have relationship, the related object will be updated or deleted as well 😭 To turn off this behavior, add cascade_delete => 0 to the $attr hashref.

⚠️ again for the people in the back: the cascaded operations are performed after the requested delete or update, so if your database has a constraint on the relationship, it will have deleted/updated the related records or raised an exception before DBIx::Class gets to perform the cascaded operation.

See "attributes" in DBIx::Class::Relationship::Base for documentation on relationship methods and valid relationship attributes. Also see DBIx::Class::ResultSet for a list of standard resultset attributes which can be assigned to relationships as well.

Note that if you supply a condition on which to join, and the column in the current table allows nulls (i.e., has the is_nullable attribute set to a true value), than might_have will warn about this because it's naughty and you shouldn't do that 🙈 The warning will look something like:

"might_have/has_one" must not be on columns with is_nullable set to true (MySchema::SomeClass/key)

If you must be naughty, you can suppress the warning by setting the DBIC_DONT_VALIDATE_RELS environment variable to a true value. Otherwise, you probably just meant to use DBIx::Class::Relationship/belongs_to.

---

👑 has_one

Arguments: $accessor_name, $related_class, $their_fk_column|\%cond|\@cond|\&cond?, \%attrs?

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

accessor_name

This argument is the name of the method you can call on a Result object to retrieve the instance of the foreign class matching this relationship. This is often called the "relation(ship) name".

Use this accessor_name in "join" in DBIx::Class::ResultSet or "prefetch" in DBIx::Class::ResultSet to join to the foreign table indicated by this relationship 🔗

related_class

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

their_fk_column

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

OR

cond

A hashref, arrayref or coderef specifying a custom join expression. For more info see "condition" in DBIx::Class::Relationship::Base.

  # 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 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 defaults to a left join 🌈

The has_one relationship should be used when a row in the table must have exactly one related row in another table. If the related row might not exist in the foreign table, use the might_have in DBIx::Class::Relationship relationship instead.

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

See "attributes" in DBIx::Class::Relationship::Base for documentation on relationship methods and valid relationship attributes. Also see DBIx::Class::ResultSet for a list of standard resultset attributes which can be assigned to relationships as well.

Note that if you supply a condition on which to join, if the column in the current table allows nulls (i.e., has the is_nullable attribute set to a true value), than warnings might apply just as with might_have in DBIx::Class::Relationship.

---

🦋 many_to_many

Arguments: $accessor_name, $link_rel_name, $foreign_rel_name, \%attrs?

many_to_many is a Relationship bridge which has a specific meaning in DBIx::Class — see the definition in the Glossary 📖

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 related_resultset and similar methods which operate on true relationships. Not the same thing, iykyk 💀

accessor_name

This argument is the name of the method you can call on a Result object to retrieve the rows matching this relationship.

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

link_rel_name

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

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).

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 $link_rel_name parameter is the name of the accessor for the has_many relationship from this table to the link table, and the $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: "roles", "add_to_roles", "set_roles", and similarly named accessors will be created for the Role class for the "actors" many_to_many relationship 👑

See "attributes" in DBIx::Class::Relationship::Base for documentation on relationship methods and valid relationship attributes. Also see DBIx::Class::ResultSet for a list of standard resultset attributes which can be assigned to relationships as well.

---

💬 FURTHER QUESTIONS?

Check the list of additional DBIC resources 🔍

---

📜 COPYRIGHT AND LICENSE

This module is free software copyright by the DBIx::Class (DBIC) authors 🫶 You can redistribute it and/or modify it under the same terms as the DBIx::Class library.

---

and that's the whole page, bestie, you made it 🎀 this little beauty shipped in perl v5.42.0 and the doc you're reading is dated 2025-01-22 — go forth and build your relationships responsibly 💜✨