| Module | ActiveRecord::Associations::ClassMethods |
| In: |
vendor/rails/activerecord/lib/active_record/associations.rb
|
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 way 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:
Don‘t create associations that have the same name as instance methods of ActiveRecord::Base. Since the association adds a method with that name to its model, it will override the inherited method and break things. For instance, attributes and connection would be bad choices for association names.
| | belongs_to |
generated methods | belongs_to | :polymorphic | has_one
----------------------------------+------------+--------------+---------
#other | X | X | X
#other=(other) | X | X | X
#build_other(attributes={}) | X | | X
#create_other(attributes={}) | X | | X
#other.create!(attributes={}) | | | X
#other.nil? | X | X |
| | | has_many
generated methods | habtm | has_many | :through
----------------------------------+-------+----------+----------
#others | X | X | X
#others=(other,other,...) | X | X |
#other_ids | X | X | X
#other_ids=(id,id,...) | X | X |
#others<< | X | X | X
#others.push | X | X | X
#others.concat | X | X | X
#others.build(attributes={}) | X | X | X
#others.create(attributes={}) | X | X |
#others.create!(attributes={}) | X | X | X
#others.size | X | X | X
#others.length | X | X | X
#others.count | | X | X
#others.sum(args*,&block) | X | X | X
#others.empty? | X | X | X
#others.clear | X | X |
#others.delete(other,other,...) | X | X | X
#others.delete_all | X | X |
#others.destroy_all | X | X | X
#others.find(*args) | X | X | X
#others.find_first | X | |
#others.uniq | X | X |
#others.reset | X | X | X
ActiveRecord associations can be used to describe relations with one-to-one, one-to-many and many-to-many cardinality. Each model uses an association to describe its role in the relation. In each case, the belongs_to association is used in the model that has the foreign key.
Use has_one in the base, and belongs_to in the associated model.
class Employee < ActiveRecord::Base
has_one :office
end
class Office < ActiveRecord::Base
belongs_to :employee # foreign key - employee_id
end
Use has_many in the base, and belongs_to in the associated model.
class Manager < ActiveRecord::Base
has_many :employees
end
class Employee < ActiveRecord::Base
belongs_to :manager # foreign key - manager_id
end
There are two ways to build a many-to-many relationship.
The first way uses a has_many association with the :through option and a join model, so there are two stages of associations.
class Assignment < ActiveRecord::Base
belongs_to :programmer # foreign key - programmer_id
belongs_to :project # foreign key - project_id
end
class Programmer < ActiveRecord::Base
has_many :assignments
has_many :projects, :through => :assignments
end
class Project < ActiveRecord::Base
has_many :assignments
has_many :programmers, :through => :assignments
end
For the second way, use has_and_belongs_to_many in both models. This requires a join table that has no corresponding model or primary key.
class Programmer < ActiveRecord::Base
has_and_belongs_to_many :projects # foreign keys in the join table
end
class Project < ActiveRecord::Base
has_and_belongs_to_many :programmers # foreign keys in the join table
end
Choosing which way to build a many-to-many relationship is not always simple. If you need to work with the relationship model as its own entity, use has_many :through. Use has_and_belongs_to_many when working with legacy schemas or when you never work directly with the relationship itself.
Both express a 1-1 relationship. The difference is mostly where to place the foreign key, which goes on the table for the class declaring the belongs_to relationship. Example:
class User < ActiveRecord::Base
# I reference an account.
belongs_to :account
end
class Account < ActiveRecord::Base
# One user references me.
has_one :user
end
The tables for these classes could look something like:
CREATE TABLE users (
id int(11) NOT NULL auto_increment,
account_id int(11) default NULL,
name varchar default NULL,
PRIMARY KEY (id)
)
CREATE TABLE accounts (
id int(11) NOT NULL auto_increment,
name varchar default NULL,
PRIMARY KEY (id)
)
You can manipulate objects and associations before they are saved to the database, but there is some special behavior you should be aware of, mostly involving the saving of associated objects.
Similar to the normal callbacks that hook into the lifecycle of an Active Record object, you can also define callbacks that get triggered when you add an object to or remove an object from an association collection. Example:
class Project
has_and_belongs_to_many :developers, :after_add => :evaluate_velocity
def evaluate_velocity(developer)
...
end
end
It‘s possible to stack callbacks by passing them as an array. Example:
class Project
has_and_belongs_to_many :developers, :after_add => [:evaluate_velocity, Proc.new { |p, d| p.shipping_date = Time.now}]
end
Possible callbacks are: before_add, after_add, before_remove and after_remove.
Should any of the before_add callbacks throw an exception, the object does not get added to the collection. Same with the before_remove callbacks; if an exception is thrown the object doesn‘t get removed.
The proxy objects that control the access to associations can be extended through anonymous modules. This is especially beneficial for adding new finders, creators, and other factory-type methods that are only used as part of this association. Example:
class Account < ActiveRecord::Base
has_many :people do
def find_or_create_by_name(name)
first_name, last_name = name.split(" ", 2)
find_or_create_by_first_name_and_last_name(first_name, last_name)
end
end
end
person = Account.find(:first).people.find_or_create_by_name("David Heinemeier Hansson")
person.first_name # => "David"
person.last_name # => "Heinemeier Hansson"
If you need to share the same extensions between many associations, you can use a named extension module. Example:
module FindOrCreateByNameExtension
def find_or_create_by_name(name)
first_name, last_name = name.split(" ", 2)
find_or_create_by_first_name_and_last_name(first_name, last_name)
end
end
class Account < ActiveRecord::Base
has_many :people, :extend => FindOrCreateByNameExtension
end
class Company < ActiveRecord::Base
has_many :people, :extend => FindOrCreateByNameExtension
end
If you need to use multiple named extension modules, you can specify an array of modules with the :extend option. In the case of name conflicts between methods in the modules, methods in modules later in the array supercede those earlier in the array. Example:
class Account < ActiveRecord::Base
has_many :people, :extend => [FindOrCreateByNameExtension, FindRecentExtension]
end
Some extensions can only be made to work with knowledge of the association proxy‘s internals. Extensions can access relevant state using accessors on the association proxy:
Has Many associations can be configured with the :through option to use an explicit join model to retrieve the data. This operates similarly to a has_and_belongs_to_many association. The advantage is that you‘re able to add validations, callbacks, and extra attributes on the join model. Consider the following schema:
class Author < ActiveRecord::Base
has_many :authorships
has_many :books, :through => :authorships
end
class Authorship < ActiveRecord::Base
belongs_to :author
belongs_to :book
end
@author = Author.find :first
@author.authorships.collect { |a| a.book } # selects all books that the author's authorships belong to.
@author.books # selects all books by using the Authorship join model
You can also go through a has_many association on the join model:
class Firm < ActiveRecord::Base
has_many :clients
has_many :invoices, :through => :clients
end
class Client < ActiveRecord::Base
belongs_to :firm
has_many :invoices
end
class Invoice < ActiveRecord::Base
belongs_to :client
end
@firm = Firm.find :first
@firm.clients.collect { |c| c.invoices }.flatten # select all invoices for all clients of the firm
@firm.invoices # selects all invoices by going through the Client join model.
Polymorphic associations on models are not restricted on what types of models they can be associated with. Rather, they specify an interface that a has_many association must adhere to.
class Asset < ActiveRecord::Base
belongs_to :attachable, :polymorphic => true
end
class Post < ActiveRecord::Base
has_many :assets, :as => :attachable # The :as option specifies the polymorphic interface to use.
end
@asset.attachable = @post
This works by using a type column in addition to a foreign key to specify the associated record. In the Asset example, you‘d need an attachable_id integer column and an attachable_type string column.
Using polymorphic associations in combination with single table inheritance (STI) is a little tricky. In order for the associations to work as expected, ensure that you store the base model for the STI models in the type column of the polymorphic association. To continue with the asset example above, suppose there are guest posts and member posts that use the posts table for STI. In this case, there must be a type column in the posts table.
class Asset < ActiveRecord::Base
belongs_to :attachable, :polymorphic => true
def attachable_type=(sType)
super(sType.to_s.classify.constantize.base_class.to_s)
end
end
class Post < ActiveRecord::Base
# because we store "Post" in attachable_type now :dependent => :destroy will work
has_many :assets, :as => :attachable, :dependent => :destroy
end
class GuestPost < Post
end
class MemberPost < Post
end
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 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 need 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.
To include a deep hierarchy of associations, use a hash:
for post in Post.find(:all, :include => [ :author, { :comments => { :author => :gravatar } } ])
That‘ll grab not only all the comments but all their authors and gravatar pictures. You can mix and match symbols, arrays and hashes in any combination to describe the associations you want to load.
All of this power shouldn‘t fool you into thinking 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 it‘s no catch-all for performance problems, but it‘s a great way to cut down on the number of queries in a situation as the one described above.
Since the eager loading pulls 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. Because eager loading generates the SELECT statement too, the :select option is ignored.
You can use eager loading on multiple associations from the same table, but you cannot use those associations in orders and conditions as there is currently not any way to disambiguate them. Eager loading will not pull additional attributes on join tables, so "rich associations" with has_and_belongs_to_many are not a good fit for eager loading.
When eager loaded, conditions are interpolated in the context of the model class, not the model instance. Conditions are lazily interpolated before the actual model exists.
ActiveRecord uses table aliasing in the case that a table is referenced multiple times in a join. If a table is referenced only once, the standard table name is used. The second time, the table is aliased as #{reflection_name}_#{parent_table_name}. Indexes are appended for any more successive uses of the table name.
Post.find :all, :include => :comments # => SELECT ... FROM posts LEFT OUTER JOIN comments ON ... Post.find :all, :include => :special_comments # STI # => SELECT ... FROM posts LEFT OUTER JOIN comments ON ... AND comments.type = 'SpecialComment' Post.find :all, :include => [:comments, :special_comments] # special_comments is the reflection name, posts is the parent table name # => SELECT ... FROM posts LEFT OUTER JOIN comments ON ... LEFT OUTER JOIN comments special_comments_posts
Acts as tree example:
TreeMixin.find :all, :include => :children
# => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
TreeMixin.find :all, :include => {:children => :parent} # using cascading eager includes
# => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
LEFT OUTER JOIN parents_mixins ...
TreeMixin.find :all, :include => {:children => {:parent => :children}}
# => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
LEFT OUTER JOIN parents_mixins ...
LEFT OUTER JOIN mixins childrens_mixins_2
Has and Belongs to Many join tables use the same idea, but add a _join suffix:
Post.find :all, :include => :categories
# => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
Post.find :all, :include => {:categories => :posts}
# => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
LEFT OUTER JOIN categories_posts posts_categories_join LEFT OUTER JOIN posts posts_categories
Post.find :all, :include => {:categories => {:posts => :categories}}
# => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
LEFT OUTER JOIN categories_posts posts_categories_join LEFT OUTER JOIN posts posts_categories
LEFT OUTER JOIN categories_posts categories_posts_join LEFT OUTER JOIN categories categories_posts
If you wish to specify your own custom joins using a :joins option, those table names will take precedence over the eager associations:
Post.find :all, :include => :comments, :joins => "inner join comments ..."
# => SELECT ... FROM posts LEFT OUTER JOIN comments_posts ON ... INNER JOIN comments ...
Post.find :all, :include => [:comments, :special_comments], :joins => "inner join comments ..."
# => SELECT ... FROM posts LEFT OUTER JOIN comments comments_posts ON ...
LEFT OUTER JOIN comments special_comments_posts ...
INNER JOIN comments ...
Table aliases are automatically truncated according to the maximum length of table identifiers according to the specific database.
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 will 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. Example:
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
If you attempt to assign an object to an association that doesn‘t match the inferred or specified :class_name, you‘ll get an ActiveRecord::AssociationTypeMismatch.
All of the association macros can be specialized through options. This makes cases more complex than the simple and guessable ones possible.
Adds the following methods for retrieval and query for a single associated object for which this object holds an id: association is replaced with the symbol passed as the first argument, so belongs_to :author would add among others author.nil?.
Example: A Post class declares belongs_to :author, which will add:
The declaration can also include an options hash to specialize the behavior of the association.
Options are:
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}'
belongs_to :attachable, :polymorphic => true
# File vendor/rails/activerecord/lib/active_record/associations.rb, line 795
795: def belongs_to(association_id, options = {})
796: reflection = create_belongs_to_reflection(association_id, options)
797:
798: if reflection.options[:polymorphic]
799: association_accessor_methods(reflection, BelongsToPolymorphicAssociation)
800:
801: module_eval do
802: before_save "association = instance_variable_get(\"@\#{reflection.name}\")\nif association && association.target\nif association.new_record?\nassociation.save(true)\nend\n\nif association.updated?\nself[\"\#{reflection.primary_key_name}\"] = association.id\nself[\"\#{reflection.options[:foreign_type]}\"] = association.class.base_class.name.to_s\nend\nend\n"
803: end
804: else
805: association_accessor_methods(reflection, BelongsToAssociation)
806: association_constructor_method(:build, reflection, BelongsToAssociation)
807: association_constructor_method(:create, reflection, BelongsToAssociation)
808:
809: module_eval do
810: before_save "association = instance_variable_get(\"@\#{reflection.name}\")\nif !association.nil?\nif association.new_record?\nassociation.save(true)\nend\n\nif association.updated?\nself[\"\#{reflection.primary_key_name}\"] = association.id\nend\nend\n"
811: end
812: end
813:
814: # Create the callbacks to update counter cache
815: if options[:counter_cache]
816: cache_column = options[:counter_cache] == true ?
817: "#{self.to_s.underscore.pluralize}_count" :
818: options[:counter_cache]
819:
820: module_eval(
821: "after_create '#{reflection.name}.class.increment_counter(\"#{cache_column}\", #{reflection.primary_key_name})" +
822: " unless #{reflection.name}.nil?'"
823: )
824:
825: module_eval(
826: "before_destroy '#{reflection.name}.class.decrement_counter(\"#{cache_column}\", #{reflection.primary_key_name})" +
827: " unless #{reflection.name}.nil?'"
828: )
829:
830: module_eval(
831: "#{reflection.class_name}.send(:attr_readonly,\"#{cache_column}\".intern) if defined?(#{reflection.class_name}) && #{reflection.class_name}.respond_to?(:attr_readonly)"
832: )
833: end
834: end
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". Note that this precedence is calculated using the < operator for String. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables paper_boxes and papers to generate a join table name of papers_paper_boxes because of the length of the name paper_boxes, but it in fact generates a join table name of paper_boxes_papers. Be aware of this caveat, and use the custom join_table option if you need to.
Deprecated: Any additional fields added to the join table will be placed as attributes when pulling records out through has_and_belongs_to_many associations. Records returned from join tables with additional attributes will be marked as ReadOnly (because we can‘t save changes to the additional attributes). It‘s strongly recommended that you upgrade any associations with attributes to a real join model (see introduction).
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?.
Example: A Developer class declares has_and_belongs_to_many :projects, which will add:
The declaration may include an options hash to specialize the behavior of the association.
Options are:
Option examples:
has_and_belongs_to_many :projects
has_and_belongs_to_many :projects, :include => [ :milestones, :manager ]
has_and_belongs_to_many :nations, :class_name => "Country"
has_and_belongs_to_many :categories, :join_table => "prods_cats"
has_and_belongs_to_many :active_projects, :join_table => 'developers_projects', :delete_sql =>
'DELETE FROM developers_projects WHERE active=1 AND developer_id = #{id} AND project_id = #{record.id}'
# File vendor/rails/activerecord/lib/active_record/associations.rb, line 951
951: def has_and_belongs_to_many(association_id, options = {}, &extension)
952: reflection = create_has_and_belongs_to_many_reflection(association_id, options, &extension)
953:
954: add_multiple_associated_save_callbacks(reflection.name)
955: collection_accessor_methods(reflection, HasAndBelongsToManyAssociation)
956:
957: # Don't use a before_destroy callback since users' before_destroy
958: # callbacks will be executed after the association is wiped out.
959: old_method = "destroy_without_habtm_shim_for_#{reflection.name}"
960: class_eval "alias_method :\#{old_method}, :destroy_without_callbacks\ndef destroy_without_callbacks\n\#{reflection.name}.clear\n\#{old_method}\nend\n" unless method_defined?(old_method)
961:
962: add_association_callbacks(reflection.name, options)
963: end
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?.
Example: A Firm class declares has_many :clients, which will add:
The declaration can also include an options hash to specialize the behavior of the association.
Options are:
Option examples:
has_many :comments, :order => "posted_on"
has_many :comments, :include => :author
has_many :people, :class_name => "Person", :conditions => "deleted = 0", :order => "name"
has_many :tracks, :order => "position", :dependent => :destroy
has_many :comments, :dependent => :nullify
has_many :tags, :as => :taggable
has_many :subscribers, :through => :subscriptions, :source => :user
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'
# File vendor/rails/activerecord/lib/active_record/associations.rb, line 666
666: def has_many(association_id, options = {}, &extension)
667: reflection = create_has_many_reflection(association_id, options, &extension)
668:
669: configure_dependency_for_has_many(reflection)
670:
671: if options[:through]
672: collection_reader_method(reflection, HasManyThroughAssociation)
673: collection_accessor_methods(reflection, HasManyThroughAssociation, false)
674: else
675: add_multiple_associated_save_callbacks(reflection.name)
676: add_association_callbacks(reflection.name, reflection.options)
677: collection_accessor_methods(reflection, HasManyAssociation)
678: end
679: end
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?.
Example: An Account class declares has_one :beneficiary, which will add:
The declaration can also include an options hash to specialize the behavior of the association.
Options are:
Option examples:
has_one :credit_card, :dependent => :destroy # destroys the associated credit card has_one :credit_card, :dependent => :nullify # updates the associated records foreign key value to NULL rather than destroying it has_one :last_comment, :class_name => "Comment", :order => "posted_on" has_one :project_manager, :class_name => "Person", :conditions => "role = 'project_manager'" has_one :attachment, :as => :attachable
# File vendor/rails/activerecord/lib/active_record/associations.rb, line 726
726: def has_one(association_id, options = {})
727: reflection = create_has_one_reflection(association_id, options)
728:
729: module_eval do
730: after_save "association = instance_variable_get(\"@\#{reflection.name}\")\nif !association.nil? && (new_record? || association.new_record? || association[\"\#{reflection.primary_key_name}\"] != id)\nassociation[\"\#{reflection.primary_key_name}\"] = id\nassociation.save(true)\nend\n"
731: end
732:
733: association_accessor_methods(reflection, HasOneAssociation)
734: association_constructor_method(:build, reflection, HasOneAssociation)
735: association_constructor_method(:create, reflection, HasOneAssociation)
736:
737: configure_dependency_for_has_one(reflection)
738: end