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:
- 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.
Association callbacks
Similiar to the normal callbacks that hook into the lifecycle of an Active Record object, you can also define callbacks that get trigged when you add an object to or removing an object from a 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.
Association extensions
The proxy objects that controls 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
Association Join Models
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
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 <tt>:as</tt> 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.
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 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.
Please note that limited eager loading with has_many and has_and_belongs_to_many associations is not compatible with describing conditions on these eager tables. This will work:
Post.find(:all, :include => :comments, :conditions => "posts.title = 'magic forest'", :limit => 2)
…but this will not (and an ArgumentError will be raised):
Post.find(:all, :include => :comments, :conditions => "comments.body like 'Normal%'", :limit => 2)
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. Eager loading will not pull additional attributes on join tables, so "rich associations" with has_and_belongs_to_many is not a good fit for eager loading.
Table Aliasing
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.
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.
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 post.author = Author.new)
- Post#create_author (similar to post.author = Author.new; post.author.save; post.author)
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). You can also specify a custom counter cache column by given that name instead of a true/false value to this option (e.g., :counter_cache => :my_custom_counter.)
- :include - specify second-order associations that should be eager loaded when this object is loaded.
- :polymorphic - specify this association is a polymorphic association by passing true.
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
[ show source ]
# File vendor/rails/activerecord/lib/active_record/associations.rb, line 647 647: def belongs_to(association_id, options = {}) 648: reflection = create_belongs_to_reflection(association_id, options) 649: 650: if reflection.options[:polymorphic] 651: association_accessor_methods(reflection, BelongsToPolymorphicAssociation) 652: 653: module_eval do 654: 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\nself[\"\#{reflection.options[:foreign_type]}\"] = association.class.base_class.name.to_s\nend\nend\n" 655: end 656: else 657: association_accessor_methods(reflection, BelongsToAssociation) 658: association_constructor_method(:build, reflection, BelongsToAssociation) 659: association_constructor_method(:create, reflection, BelongsToAssociation) 660: 661: module_eval do 662: 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" 663: end 664: 665: # deprecated api 666: deprecated_has_association_method(reflection.name) 667: deprecated_association_comparison_method(reflection.name, reflection.class_name) 668: end 669: 670: if options[:counter_cache] 671: cache_column = options[:counter_cache] == true ? 672: "#{self.to_s.underscore.pluralize}_count" : 673: options[:counter_cache] 674: 675: module_eval( 676: "after_create '#{reflection.name}.class.increment_counter(\"#{cache_column}\", #{reflection.primary_key_name})" + 677: " unless #{reflection.name}.nil?'" 678: ) 679: 680: module_eval( 681: "before_destroy '#{reflection.name}.class.decrement_counter(\"#{cache_column}\", #{reflection.primary_key_name})" + 682: " unless #{reflection.name}.nil?'" 683: ) 684: end 685: 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".
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 attrbutes). 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?.
- 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). This method is now deprecated.
- 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=objects - replaces the collections content by deleting and adding objects as appropriate.
- collection_singular_ids=ids - replace the collection by the objects identified by the primary keys in ids
- 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=
- Developer#project_ids=
- 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 if the associated class is Project, the 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
- :extend - anonymous module for extending the proxy, see "Association extensions".
- :include - specify second-order associations that should be eager loaded when the collection is loaded.
- :group: An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.
- :limit: An integer determining the limit on the number of rows that should be returned.
- :offset: An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
- :select: By default, this is * as in SELECT * FROM, but can be changed if you for example want to do a join, but not include the joined columns.
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}'
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# File vendor/rails/activerecord/lib/active_record/associations.rb, line 792 792: def has_and_belongs_to_many(association_id, options = {}, &extension) 793: reflection = create_has_and_belongs_to_many_reflection(association_id, options, &extension) 794: 795: add_multiple_associated_save_callbacks(reflection.name) 796: collection_accessor_methods(reflection, HasAndBelongsToManyAssociation) 797: 798: # Don't use a before_destroy callback since users' before_destroy 799: # callbacks will be executed after the association is wiped out. 800: old_method = "destroy_without_habtm_shim_for_#{reflection.name}" 801: class_eval "alias_method :\#{old_method}, :destroy_without_callbacks\ndef destroy_without_callbacks\n\#{reflection.name}.clear\n\#{old_method}\nend\n" 802: 803: add_association_callbacks(reflection.name, options) 804: 805: # deprecated api 806: deprecated_collection_count_method(reflection.name) 807: deprecated_add_association_relation(reflection.name) 808: deprecated_remove_association_relation(reflection.name) 809: deprecated_has_collection_method(reflection.name) 810: 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?.
- 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=objects - replaces the collections content by deleting and adding objects as appropriate.
- collection_singular_ids=ids - replace the collection by the objects identified by the primary keys in ids
- collection.clear - removes every object from the collection. This destroys the associated objects if they are :dependent, deletes them directly from the database if they are :dependent => :delete_all, and sets their foreign keys to NULL otherwise.
- 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 it’s 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 it’s 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=
- Firm#client_ids=
- 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("firm_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"
- :group - specify the attribute by which the associated objects are returned as a "GROUP BY" sql fragment, such as "category"
- :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 :destroy all the associated objects are destroyed alongside this object by calling their destroy method. If set to :delete_all all associated objects are deleted without calling their destroy method. If set to :nullify all associated objects’ foreign keys are set to NULL without calling their save callbacks. NOTE: :dependent => true is deprecated and has been replaced with :dependent => :destroy. May not be set if :exclusively_dependent is also set.
- :exclusively_dependent - Deprecated; equivalent to :dependent => :delete_all. 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 depend 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.
- :extend - specify a named module for extending the proxy, see "Association extensions".
- :include - specify second-order associations that should be eager loaded when the collection is loaded.
- :group: An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.
- :limit: An integer determining the limit on the number of rows that should be returned.
- :offset: An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
- :select: By default, this is * as in SELECT * FROM, but can be changed if you for example want to do a join, but not include the joined columns.
- :as: Specifies a polymorphic interface (See belongs_to).
- :through: Specifies a Join Model to perform the query through. Options for :class_name and :foreign_key are ignored, as the association uses the source reflection. You can only use a :through query through a belongs_to or has_many association.
- :source: Specifies the source association name used by has_many :through queries. Only use it if the name cannot be inferred from the association. has_many :subscribers, :through => :subscriptions will look for either +:subscribers+ or +:subscriber+ on Subscription, unless a +:source+ is given.
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'
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# File vendor/rails/activerecord/lib/active_record/associations.rb, line 519 519: def has_many(association_id, options = {}, &extension) 520: reflection = create_has_many_reflection(association_id, options, &extension) 521: 522: configure_dependency_for_has_many(reflection) 523: 524: if options[:through] 525: collection_reader_method(reflection, HasManyThroughAssociation) 526: else 527: add_multiple_associated_save_callbacks(reflection.name) 528: add_association_callbacks(reflection.name, reflection.options) 529: collection_accessor_methods(reflection, HasManyAssociation) 530: end 531: 532: add_deprecated_api_for_has_many(reflection.name) 533: 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?.
- 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, :conditions => "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 :destroy (or true) all the associated objects are destroyed when this object is. Also, 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.
- :include - specify second-order associations that should be eager loaded when this object is loaded.
Option examples:
has_one :credit_card, :dependent => :destroy # destroys the associated credit card has_one :credit_card, :dependent => :nullify # updates the associated records foriegn 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'"
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# File vendor/rails/activerecord/lib/active_record/associations.rb, line 577 577: def has_one(association_id, options = {}) 578: reflection = create_has_one_reflection(association_id, options) 579: 580: module_eval do 581: after_save "association = instance_variable_get(\"@\#{reflection.name}\")\nunless association.nil?\nassociation[\"\#{reflection.primary_key_name}\"] = id\nassociation.save(true)\nend\n" 582: end 583: 584: association_accessor_methods(reflection, HasOneAssociation) 585: association_constructor_method(:build, reflection, HasOneAssociation) 586: association_constructor_method(:create, reflection, HasOneAssociation) 587: 588: configure_dependency_for_has_one(reflection) 589: 590: # deprecated api 591: deprecated_has_association_method(reflection.name) 592: deprecated_association_comparison_method(reflection.name, reflection.class_name) 593: end