Associations are a set of macro-like class methods for tying objects together through foreign keys. They express relationships like "Project has one Project Manager" or "Project belongs to a Portfolio". Each macro adds a number of methods to the class which are specialized according to the collection or association symbol and the options hash. It works much the same was as Ruby’s own attr* methods. Example: 

  class Project < ActiveRecord::Base
    belongs_to              :portfolio
    has_one                 :project_manager
    has_many                :milestones
    has_and_belongs_to_many :categories
  end

The project class now has the following methods (and more) to ease the traversal and manipulation of its relationships:

  • Project#portfolio, Project#portfolio=(portfolio), Project#portfolio.nil?
  • Project#project_manager, Project#project_manager=(project_manager), Project#project_manager.nil?,
  • Project#milestones.empty?, Project#milestones.size, Project#milestones, Project#milestones<<(milestone), Project#milestones.delete(milestone), Project#milestones.find(milestone_id), Project#milestones.find_all(conditions), Project#milestones.build, Project#milestones.create
  • Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1), Project#categories.delete(category1)

Example

Is it belongs_to or has_one?

Both express a 1-1 relationship, the difference is mostly where to place the foreign key, which goes on the table for the class saying belongs_to. Example: 

  class Post < ActiveRecord::Base
    has_one :author
  end

  class Author < ActiveRecord::Base
    belongs_to :post
  end

The tables for these classes could look something like: 

  CREATE TABLE posts (
    id int(11) NOT NULL auto_increment,
    title varchar default NULL,
    PRIMARY KEY  (id)
  )

  CREATE TABLE authors (
    id int(11) NOT NULL auto_increment,
    post_id int(11) default NULL,
    name varchar default NULL,
    PRIMARY KEY  (id)
  )

Unsaved objects and associations

You can manipulate objects and associations before they are saved to the database, but there is some special behaviour you should be aware of, mostly involving the saving of associated objects.

One-to-one associations

  • Assigning an object to a has_one association automatically saves that object, and the object being replaced (if there is one), in order to update their primary keys - except if the parent object is unsaved (new_record? == true).
  • If either of these saves fail (due to one of the objects being invalid) the assignment statement returns false and the assignment is cancelled.
  • If you wish to assign an object to a has_one association without saving it, use the association.build method (documented below).
  • Assigning an object to a belongs_to association does not save the object, since the foreign key field belongs on the parent. It does not save the parent either.

Collections

  • Adding an object to a collection (has_many or has_and_belongs_to_many) automatically saves that object, except if the parent object (the owner of the collection) is not yet stored in the database.
  • If saving any of the objects being added to a collection (via push or similar) fails, then push returns false.
  • You can add an object to a collection without automatically saving it by using the collection.build method (documented below).
  • All unsaved (new_record? == true) members of the collection are automatically saved when the parent is saved.

Caching

All of the methods are built on a simple caching principle that will keep the result of the last query around unless specifically instructed not to. The cache is even shared across methods to make it even cheaper to use the macro-added methods without worrying too much about performance at the first go. Example: 

  project.milestones             # fetches milestones from the database
  project.milestones.size        # uses the milestone cache
  project.milestones.empty?      # uses the milestone cache
  project.milestones(true).size  # fetches milestones from the database
  project.milestones             # uses the milestone cache

Eager loading of associations

Eager loading is a way to find objects of a certain class and a number of named associations along with it in a single SQL call. This is one of the easiest ways of to prevent the dreaded 1+N problem in which fetching 100 posts that each needs to display their author triggers 101 database queries. Through the use of eager loading, the 101 queries can be reduced to 1. Example: 

  class Post < ActiveRecord::Base
    belongs_to :author
    has_many   :comments
  end

Consider the following loop using the class above: 

  for post in Post.find(:all)
    puts "Post:            " + post.title
    puts "Written by:      " + post.author.name
    puts "Last comment on: " + post.comments.first.created_on
  end

To iterate over these one hundred posts, we’ll generate 201 database queries. Let’s first just optimize it for retrieving the author: 

  for post in Post.find(:all, :include => :author)

This references the name of the belongs_to association that also used the :author symbol, so the find will now weave in a join something like this: LEFT OUTER JOIN authors ON authors.id = posts.author_id. Doing so will cut down the number of queries from 201 to 101.

We can improve upon the situation further by referencing both associations in the finder with: 

  for post in Post.find(:all, :include => [ :author, :comments ])

That‘ll add another join along the lines of: LEFT OUTER JOIN comments ON comments.post_id = posts.id. And we’ll be down to 1 query. But that shouldn’t fool you to think that you can pull out huge amounts of data with no performance penalty just because you’ve reduced the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So its no catch-all for performance problems, but its a great way to cut down on the number of queries in a situation as the one described above.

Please note that because eager loading is fetching both models and associations in the same grab, it doesn’t make sense to use the :limit property and it will be ignored if attempted.

Also have in mind that since the eager loading is pulling from multiple tables, you’ll have to disambiguate any column references in both conditions and orders. So :order => "posts.id DESC" will work while :order => "id DESC" will not. This may require that you alter the :order and :conditions on the association definitions themselves.

It’s currently not possible to use eager loading on multiple associations from the same table.

Modules

By default, associations will look for objects within the current module scope. Consider: 

  module MyApplication
    module Business
      class Firm < ActiveRecord::Base
         has_many :clients
       end

      class Company < ActiveRecord::Base; end
    end
  end

When Firm#clients is called, it’ll in turn call MyApplication::Business::Company.find(firm.id). If you want to associate with a class in another module scope this can be done by specifying the complete class name, such as: 

  module MyApplication
    module Business
      class Firm < ActiveRecord::Base; end
    end

    module Billing
      class Account < ActiveRecord::Base
        belongs_to :firm, :class_name => "MyApplication::Business::Firm"
      end
    end
  end

Type safety with ActiveRecord::AssociationTypeMismatch

If you attempt to assign an object to an association that doesn’t match the inferred or specified :class_name, you’ll get a ActiveRecord::AssociationTypeMismatch.

Options

All of the association macros can be specialized through options which makes more complex cases than the simple and guessable ones possible.

Methods
Public Instance methods
belongs_to(association_id, options = {})

Adds the following methods for retrieval and query for a single associated object that this object holds an id to. association is replaced with the symbol passed as the first argument, so belongs_to :author would add among others author.nil?.

  • association(force_reload = false) - returns the associated object. Nil is returned if none is found.
  • association=(associate) - assigns the associate object, extracts the primary key, and sets it as the foreign key.
  • association.nil? - returns true if there is no associated object.
  • build_association(attributes = {}) - returns a new object of the associated type that has been instantiated with attributes and linked to this object through a foreign key but has not yet been saved.
  • create_association(attributes = {}) - returns a new object of the associated type that has been instantiated with attributes and linked to this object through a foreign key and that has already been saved (if it passed the validation).

Example: A Post class declares belongs_to :author, which will add:

  • Post#author (similar to Author.find(author_id))
  • Post#author=(author) (similar to post.author_id = author.id)
  • Post#author? (similar to post.author == some_author)
  • Post#author.nil?
  • Post#build_author (similar to Author.new("post_id" => id))
  • Post#create_author (similar to b = Author.new("post_id" => id); b.save; b)

The declaration can also include an options hash to specialize the behavior of the association.

Options are:

  • :class_name - specify the class name of the association. Use it only if that name can’t be inferred from the association name. So has_one :author will by default be linked to the Author class, but if the real class name is Person, you’ll have to specify it with this option.
  • :conditions - specify the conditions that the associated object must meet in order to be included as a "WHERE" sql fragment, such as "authorized = 1".
  • :order - specify the order from which the associated object will be picked at the top. Specified as an "ORDER BY" sql fragment, such as "last_name, first_name DESC"
  • :foreign_key - specify the foreign key used for the association. By default this is guessed to be the name of the associated class in lower-case and "_id" suffixed. So a Person class that makes a belongs_to association to a Boss class will use "boss_id" as the default foreign_key.
  • :counter_cache - caches the number of belonging objects on the associate class through use of increment_counter and decrement_counter. The counter cache is incremented when an object of this class is created and decremented when it’s destroyed. This requires that a column named "#{table_name}_count" (such as comments_count for a belonging Comment class) is used on the associate class (such as a Post class).

Option examples: 

  belongs_to :firm, :foreign_key => "client_of"
  belongs_to :author, :class_name => "Person", :foreign_key => "author_id"
  belongs_to :valid_coupon, :class_name => "Coupon", :foreign_key => "coupon_id",
             :conditions => 'discounts > #{payments_count}'

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/associations.rb, line 400
400:       def belongs_to(association_id, options = {})
401:         validate_options([ :class_name, :foreign_key, :remote, :conditions, :order, :dependent, :counter_cache ], options.keys)
402: 
403:         association_name, association_class_name, class_primary_key_name =
404:             associate_identification(association_id, options[:class_name], options[:foreign_key], false)
405: 
406:         require_association_class(association_class_name)
407: 
408:         association_class_primary_key_name = options[:foreign_key] || Inflector.underscore(Inflector.demodulize(association_class_name)) + "_id"
409: 
410:         association_accessor_methods(association_name, association_class_name, association_class_primary_key_name, options, BelongsToAssociation)
411:         association_constructor_method(:build, association_name, association_class_name, association_class_primary_key_name, options, BelongsToAssociation)
412:         association_constructor_method(:create, association_name, association_class_name, association_class_primary_key_name, options, BelongsToAssociation)
413: 
414:         module_eval do
415:           before_save "association = instance_variable_get(\"@\#{association_name}\")\nif not association.nil? and association.new_record?\nassociation.save(true)\nself[\"\#{association_class_primary_key_name}\"] = association.id\nassociation.send(:construct_sql)\nend\n"
416:         end
417:       
418:         if options[:counter_cache]
419:           module_eval(
420:             "after_create '#{association_class_name}.increment_counter(\"#{self.to_s.underscore.pluralize + "_count"}\", #{association_class_primary_key_name})" +
421:             " unless #{association_name}.nil?'"
422:           )
423: 
424:           module_eval(
425:             "before_destroy '#{association_class_name}.decrement_counter(\"#{self.to_s.underscore.pluralize + "_count"}\", #{association_class_primary_key_name})" +
426:             " unless #{association_name}.nil?'"
427:           )          
428:         end
429: 
430:         # deprecated api
431:         deprecated_has_association_method(association_name)
432:         deprecated_association_comparison_method(association_name, association_class_name)
433:       end
has_and_belongs_to_many(association_id, options = {})

Associates two classes via an intermediate join table. Unless the join table is explicitly specified as an option, it is guessed using the lexical order of the class names. So a join between Developer and Project will give the default join table name of "developers_projects" because "D" outranks "P".

Any additional fields added to the join table will be placed as attributes when pulling records out through has_and_belongs_to_many associations. This is helpful when have information about the association itself that you want available on retrieval. Note that any fields in the join table will override matching field names in the two joined tables. As a consequence, having an "id" field in the join table usually has the undesirable result of clobbering the "id" fields in either of the other two tables.

Adds the following methods for retrieval and query. collection is replaced with the symbol passed as the first argument, so has_and_belongs_to_many :categories would add among others categories.empty?.

  • collection(force_reload = false) - returns an array of all the associated objects. An empty array is returned if none is found.
  • collection<<(object, …) - adds one or more objects to the collection by creating associations in the join table (collection.push and collection.concat are aliases to this method).
  • collection.push_with_attributes(object, join_attributes) - adds one to the collection by creating an association in the join table that also holds the attributes from join_attributes (should be a hash with the column names as keys). This can be used to have additional attributes on the join, which will be injected into the associated objects when they are retrieved through the collection. (collection.concat_with_attributes is an alias to this method).
  • collection.delete(object, …) - removes one or more objects from the collection by removing their associations from the join table. This does not destroy the objects.
  • collection.clear - removes every object from the collection. This does not destroy the objects.
  • collection.empty? - returns true if there are no associated objects.
  • collection.size - returns the number of associated objects.
  • collection.find(id) - finds an associated object responding to the id and that meets the condition that it has to be associated with this object.

Example: An Developer class declares has_and_belongs_to_many :projects, which will add:

  • Developer#projects
  • Developer#projects<<
  • Developer#projects.push_with_attributes
  • Developer#projects.delete
  • Developer#projects.clear
  • Developer#projects.empty?
  • Developer#projects.size
  • Developer#projects.find(id)

The declaration may include an options hash to specialize the behavior of the association.

Options are:

  • :class_name - specify the class name of the association. Use it only if that name can’t be inferred from the association name. So has_and_belongs_to_many :projects will by default be linked to the Project class, but if the real class name is SuperProject, you’ll have to specify it with this option.
  • :join_table - specify the name of the join table if the default based on lexical order isn’t what you want. WARNING: If you’re overwriting the table name of either class, the table_name method MUST be declared underneath any has_and_belongs_to_many declaration in order to work.
  • :foreign_key - specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a has_and_belongs_to_many association will use "person_id" as the default foreign_key.
  • :association_foreign_key - specify the association foreign key used for the association. By default this is guessed to be the name of the associated class in lower-case and "_id" suffixed. So the associated class is Project that makes a has_and_belongs_to_many association will use "project_id" as the default association foreign_key.
  • :conditions - specify the conditions that the associated object must meet in order to be included as a "WHERE" sql fragment, such as "authorized = 1".
  • :order - specify the order in which the associated objects are returned as a "ORDER BY" sql fragment, such as "last_name, first_name DESC"
  • :uniq - if set to true, duplicate associated objects will be ignored by accessors and query methods
  • :finder_sql - overwrite the default generated SQL used to fetch the association with a manual one
  • :delete_sql - overwrite the default generated SQL used to remove links between the associated classes with a manual one
  • :insert_sql - overwrite the default generated SQL used to add links between the associated classes with a manual one

Option examples: 

  has_and_belongs_to_many :projects
  has_and_belongs_to_many :nations, :class_name => "Country"
  has_and_belongs_to_many :categories, :join_table => "prods_cats"

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/associations.rb, line 511
511:       def has_and_belongs_to_many(association_id, options = {})
512:         validate_options([ :class_name, :table_name, :foreign_key, :association_foreign_key, :conditions,
513:                            :join_table, :finder_sql, :delete_sql, :insert_sql, :order, :uniq ], options.keys)
514:         association_name, association_class_name, association_class_primary_key_name =
515:               associate_identification(association_id, options[:class_name], options[:foreign_key])
516: 
517:         require_association_class(association_class_name)
518: 
519:         options[:join_table] ||= join_table_name(undecorated_table_name(self.to_s), undecorated_table_name(association_class_name))
520: 
521:         add_multiple_associated_save_callbacks(association_name)
522:       
523:         collection_accessor_methods(association_name, association_class_name, association_class_primary_key_name, options, HasAndBelongsToManyAssociation)
524: 
525:         before_destroy_sql = "DELETE FROM #{options[:join_table]} WHERE #{association_class_primary_key_name} = \\\#{self.quoted_id}"
526:         module_eval(%{before_destroy "self.connection.delete(%{#{before_destroy_sql}})"}) # "
527:         
528:         # deprecated api
529:         deprecated_collection_count_method(association_name)
530:         deprecated_add_association_relation(association_name)
531:         deprecated_remove_association_relation(association_name)
532:         deprecated_has_collection_method(association_name)
533:       end
has_many(association_id, options = {})

Adds the following methods for retrieval and query of collections of associated objects. collection is replaced with the symbol passed as the first argument, so has_many :clients would add among others clients.empty?.

  • collection(force_reload = false) - returns an array of all the associated objects. An empty array is returned if none are found.
  • collection<<(object, …) - adds one or more objects to the collection by setting their foreign keys to the collection’s primary key.
  • collection.delete(object, …) - removes one or more objects from the collection by setting their foreign keys to NULL. This will also destroy the objects if they’re declared as belongs_to and dependent on this model.
  • collection.clear - removes every object from the collection. This does not destroy the objects.
  • collection.empty? - returns true if there are no associated objects.
  • collection.size - returns the number of associated objects.
  • collection.find - finds an associated object according to the same rules as Base.find.
  • collection.build(attributes = {}) - returns a new object of the collection type that has been instantiated with attributes and linked to this object through a foreign key but has not yet been saved. *Note:* This only works if an associated object already exists, not if its nil!
  • collection.create(attributes = {}) - returns a new object of the collection type that has been instantiated with attributes and linked to this object through a foreign key and that has already been saved (if it passed the validation). *Note:* This only works if an associated object already exists, not if its nil!

Example: A Firm class declares has_many :clients, which will add:

  • Firm#clients (similar to Clients.find :all, :conditions => "firm_id = #{id}")
  • Firm#clients<<
  • Firm#clients.delete
  • Firm#clients.clear
  • Firm#clients.empty? (similar to firm.clients.size == 0)
  • Firm#clients.size (similar to Client.count "firm_id = #{id}")
  • Firm#clients.find (similar to Client.find(id, :conditions => "firm_id = #{id}"))
  • Firm#clients.build (similar to Client.new("firm_id" => id))
  • Firm#clients.create (similar to c = Client.new("client_id" => id); c.save; c)

The declaration can also include an options hash to specialize the behavior of the association.

Options are:

  • :class_name - specify the class name of the association. Use it only if that name can’t be inferred from the association name. So has_many :products will by default be linked to the Product class, but if the real class name is SpecialProduct, you’ll have to specify it with this option.
  • :conditions - specify the conditions that the associated objects must meet in order to be included as a "WHERE" sql fragment, such as "price > 5 AND name LIKE ‘B%’".
  • :order - specify the order in which the associated objects are returned as a "ORDER BY" sql fragment, such as "last_name, first_name DESC"
  • :foreign_key - specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a has_many association will use "person_id" as the default foreign_key.
  • :dependent - if set to true all the associated object are destroyed alongside this object. May not be set if :exclusively_dependent is also set.
  • :exclusively_dependent - if set to true all the associated object are deleted in one SQL statement without having their before_destroy callback run. This should only be used on associations that depend solely on this class and don’t need to do any clean-up in before_destroy. The upside is that it’s much faster, especially if there’s a counter_cache involved. May not be set if :dependent is also set.
  • :finder_sql - specify a complete SQL statement to fetch the association. This is a good way to go for complex associations that depends on multiple tables. Note: When this option is used, find_in_collection is not added.
  • :counter_sql - specify a complete SQL statement to fetch the size of the association. If +:finder_sql+ is specified but +:counter_sql+, +:counter_sql+ will be generated by replacing SELECT … FROM with SELECT COUNT(*) FROM.

Option examples: 

  has_many :comments, :order => "posted_on"
  has_many :people, :class_name => "Person", :conditions => "deleted = 0", :order => "name"
  has_many :tracks, :order => "position", :dependent => true
  has_many :subscribers, :class_name => "Person", :finder_sql =>
      'SELECT DISTINCT people.* ' +
      'FROM people p, post_subscriptions ps ' +
      'WHERE ps.post_id = #{id} AND ps.person_id = p.id ' +
      'ORDER BY p.first_name'

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/associations.rb, line 256
256:       def has_many(association_id, options = {})
257:         validate_options([ :foreign_key, :class_name, :exclusively_dependent, :dependent, :conditions, :order, :finder_sql, :counter_sql ], options.keys)
258:         association_name, association_class_name, association_class_primary_key_name =
259:               associate_identification(association_id, options[:class_name], options[:foreign_key])
260:  
261:         require_association_class(association_class_name)
262: 
263:         if options[:dependent] and options[:exclusively_dependent]
264:           raise ArgumentError, ':dependent and :exclusively_dependent are mutually exclusive options.  You may specify one or the other.' # ' ruby-mode
265:         # See HasManyAssociation#delete_records.  Dependent associations
266:         # delete children, otherwise foreign key is set to NULL.
267:         elsif options[:dependent]
268:           module_eval "before_destroy '#{association_name}.each { |o| o.destroy }'"
269:         elsif options[:exclusively_dependent]
270:           module_eval "before_destroy { |record| #{association_class_name}.delete_all(%(#{association_class_primary_key_name} = \#{record.quoted_id})) }"
271:         end
272: 
273:         add_multiple_associated_save_callbacks(association_name)
274: 
275:         collection_accessor_methods(association_name, association_class_name, association_class_primary_key_name, options, HasManyAssociation)
276:         
277:         # deprecated api
278:         deprecated_collection_count_method(association_name)
279:         deprecated_add_association_relation(association_name)
280:         deprecated_remove_association_relation(association_name)
281:         deprecated_has_collection_method(association_name)
282:         deprecated_find_in_collection_method(association_name)
283:         deprecated_find_all_in_collection_method(association_name)
284:         deprecated_collection_create_method(association_name)
285:         deprecated_collection_build_method(association_name)
286:       end
has_one(association_id, options = {})

Adds the following methods for retrieval and query of a single associated object. association is replaced with the symbol passed as the first argument, so has_one :manager would add among others manager.nil?.

  • association(force_reload = false) - returns the associated object. Nil is returned if none is found.
  • association=(associate) - assigns the associate object, extracts the primary key, sets it as the foreign key, and saves the associate object.
  • association.nil? - returns true if there is no associated object.
  • build_association(attributes = {}) - returns a new object of the associated type that has been instantiated with attributes and linked to this object through a foreign key but has not yet been saved. Note: This ONLY works if an association already exists. It will NOT work if the association is nil.
  • create_association(attributes = {}) - returns a new object of the associated type that has been instantiated with attributes and linked to this object through a foreign key and that has already been saved (if it passed the validation).

Example: An Account class declares has_one :beneficiary, which will add:

  • Account#beneficiary (similar to Beneficiary.find_first "account_id = #{id}")
  • Account#beneficiary=(beneficiary) (similar to beneficiary.account_id = account.id; beneficiary.save)
  • Account#beneficiary.nil?
  • Account#build_beneficiary (similar to Beneficiary.new("account_id" => id))
  • Account#create_beneficiary (similar to b = Beneficiary.new("account_id" => id); b.save; b)

The declaration can also include an options hash to specialize the behavior of the association.

Options are:

  • :class_name - specify the class name of the association. Use it only if that name can’t be inferred from the association name. So has_one :manager will by default be linked to the Manager class, but if the real class name is Person, you’ll have to specify it with this option.
  • :conditions - specify the conditions that the associated object must meet in order to be included as a "WHERE" sql fragment, such as "rank = 5".
  • :order - specify the order from which the associated object will be picked at the top. Specified as 
     an "ORDER BY" sql fragment, such as "last_name, first_name DESC"
  • :dependent - if set to true, the associated object is destroyed when this object is. It’s also destroyed if another association is assigned.
  • :foreign_key - specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a has_one association will use "person_id" as the default foreign_key.

Option examples: 

  has_one :credit_card, :dependent => true
  has_one :last_comment, :class_name => "Comment", :order => "posted_on"
  has_one :project_manager, :class_name => "Person", :conditions => "role = 'project_manager'"

[ show source ]

     # File vendor/rails/activerecord/lib/active_record/associations.rb, line 328
328:       def has_one(association_id, options = {})
329:         validate_options([ :class_name, :foreign_key, :remote, :conditions, :order, :dependent, :counter_cache ], options.keys)
330: 
331:         association_name, association_class_name, association_class_primary_key_name =
332:             associate_identification(association_id, options[:class_name], options[:foreign_key], false)
333: 
334:         require_association_class(association_class_name)
335: 
336:         module_eval do
337:           after_save "association = instance_variable_get(\"@\#{association_name}\")\nunless association.nil?\nassociation[\"\#{association_class_primary_key_name}\"] = id\nassociation.save(true)\nassociation.send(:construct_sql)\nend\n"
338:         end
339:       
340:         association_accessor_methods(association_name, association_class_name, association_class_primary_key_name, options, HasOneAssociation)
341:         association_constructor_method(:build, association_name, association_class_name, association_class_primary_key_name, options, HasOneAssociation)
342:         association_constructor_method(:create, association_name, association_class_name, association_class_primary_key_name, options, HasOneAssociation)
343:         
344:         module_eval "before_destroy '#{association_name}.destroy unless #{association_name}.nil?'" if options[:dependent]
345: 
346:         # deprecated api
347:         deprecated_has_association_method(association_name)
348:         deprecated_association_comparison_method(association_name, association_class_name)
349:       end