Module: ActiveRecord::Associations::ClassMethods
- Defined in:
- lib/active_record/associations.rb
Overview
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, options),
Project#milestones.build, Project#milestones.create
-
Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1),
Project#categories.delete(category1)
A word of warning
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.
Auto-generated methods
Singular associations (one-to-one)
| | 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 |
Collection associations (one-to-many / many-to-many)
| | | 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 |
#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
Cardinality and associations
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.
One-to-one
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
One-to-many
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
Many-to-many
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.
Is it a belongs_to
or has_one
association?
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)
)
Unsaved objects and associations
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.
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
orhas_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
returnsfalse
. -
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
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.
Association extensions
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:
-
proxy_owner
- Returns the object the association is part of. -
proxy_reflection
- Returns the reflection object that describes the association. -
proxy_target
- Returns the associated object forbelongs_to
andhas_one
, or the collection of associated objects forhas_many
andhas_and_belongs_to_many
.
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..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 :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
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 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..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.
Eager loading is not possible with polymorphic associations. Given
class Address < ActiveRecord::Base
belongs_to :addressable, :polymorphic => true
end
a call that tries to eager load the addressable model
Address.find(:all, :include => :addressable) # INVALID
will raise ActiveRecord::EagerLoadPolymorphicError
. The reason is that the parent model’s type is a column value so its corresponding table name cannot be put in the FROM/JOIN clauses of that early query.
It does work the other way around though: if the User
model is addressable
you can eager load their addresses with :include
just fine, every piece needed to construct the query is known beforehand.
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 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
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 an ActiveRecord::AssociationTypeMismatch
.
Options
All of the association macros can be specialized through options. This makes cases more complex than the simple and guessable ones possible.
Defined Under Namespace
Classes: InnerJoinDependency, JoinDependency
Instance Method Summary collapse
-
#belongs_to(association_id, options = {}) ⇒ Object
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, sobelongs_to :author
would add among othersauthor.nil?
. -
#has_and_belongs_to_many(association_id, options = {}, &extension) ⇒ Object
Associates two classes via an intermediate join table.
-
#has_many(association_id, options = {}, &extension) ⇒ Object
Adds the following methods for retrieval and query of collections of associated objects:
collection
is replaced with the symbol passed as the first argument, sohas_many :clients
would add among othersclients.empty?
. -
#has_one(association_id, options = {}) ⇒ Object
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.
Instance Method Details
#belongs_to(association_id, options = {}) ⇒ Object
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?
.
-
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?
- returnstrue
if there is no associated object. -
build_association(attributes = {})
- returns a new object of the associated type that has been instantiated withattributes
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 withattributes
, 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 toAuthor.find(author_id)
) -
Post#author=(author)
(similar topost.author_id = author.id
) -
Post#author?
(similar topost.author == some_author
) -
Post#author.nil?
-
Post#build_author
(similar topost.author = Author.new
) -
Post#create_author
(similar topost.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. Sohas_one :author
will by default be linked to theAuthor
class, but if the real class name isPerson
, 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 aWHERE
SQL fragment, such asauthorized = 1
. -
:order
- specify the order in which the associated objects are returned as anORDER BY
SQL fragment, such aslast_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 association with an_id
suffix. So a class that defines a belongs_to :person association will useperson_id
as the defaultforeign_key
. Similarly, belongs_to :favorite_person, :class_name => “Person” will use a foreign key offavorite_person_id
. -
:counter_cache
- caches the number of belonging objects on the associate class through the use ofincrement_counter
anddecrement_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 ascomments_count
for a belongingComment
class) is used on the associate class (such as aPost
class). You can also specify a custom counter cache column by providing a column name instead of atrue
/false
value to this option (e.g.,:counter_cache => :my_custom_counter
.) Note: Specifying a counter_cache will add it to that model’s list of readonly attributes using #attr_readonly. -
:include
- specify second-order associations that should be eager loaded when this object is loaded. Not allowed if the association is polymorphic. -
:polymorphic
- specify this association is a polymorphic association by passingtrue
. Note: If you’ve enabled the counter cache, then you may want to add the counter cache attribute to the attr_readonly list in the associated classes (e.g. class Post; attr_readonly :comments_count; end).
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
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# File 'lib/active_record/associations.rb', line 813 def belongs_to(association_id, = {}) reflection = create_belongs_to_reflection(association_id, ) ivar = "@#{reflection.name}" if reflection.[:polymorphic] association_accessor_methods(reflection, BelongsToPolymorphicAssociation) module_eval do before_save <<-EOF association = instance_variable_get("#{ivar}") if instance_variable_defined?("#{ivar}") if association && association.target if association.new_record? association.save(true) end if association.updated? self["#{reflection.primary_key_name}"] = association.id self["#{reflection.[:foreign_type]}"] = association.class.base_class.name.to_s end end EOF end else association_accessor_methods(reflection, BelongsToAssociation) association_constructor_method(:build, reflection, BelongsToAssociation) association_constructor_method(:create, reflection, BelongsToAssociation) module_eval do before_save <<-EOF association = instance_variable_get("#{ivar}") if instance_variable_defined?("#{ivar}") if !association.nil? if association.new_record? association.save(true) end if association.updated? self["#{reflection.primary_key_name}"] = association.id end end EOF end end # Create the callbacks to update counter cache if [:counter_cache] cache_column = [:counter_cache] == true ? "#{self.to_s.underscore.pluralize}_count" : [:counter_cache] module_eval( "after_create '#{reflection.name}.class.increment_counter(\"#{cache_column}\", #{reflection.primary_key_name})" + " unless #{reflection.name}.nil?'" ) module_eval( "before_destroy '#{reflection.name}.class.decrement_counter(\"#{cache_column}\", #{reflection.primary_key_name})" + " unless #{reflection.name}.nil?'" ) module_eval( "#{reflection.class_name}.send(:attr_readonly,\"#{cache_column}\".intern) if defined?(#{reflection.class_name}) && #{reflection.class_name}.respond_to?(:attr_readonly)" ) end end |
#has_and_belongs_to_many(association_id, options = {}, &extension) ⇒ Object
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?
.
-
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 creating associations in the join table (collection.push
andcollection.concat
are aliases 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=objects
- replaces the collection’s content by deleting and adding objects as appropriate. -
collection_singular_ids
- returns an array of the associated objects’ ids -
collection_singular_ids=ids
- replace the collection by the objects identified by the primary keys inids
-
collection.clear
- removes every object from the collection. This does not destroy the objects. -
collection.empty?
- returnstrue
if there are no associated objects. -
collection.size
- returns the number of associated objects. -
collection.find(id)
- finds an associated object responding to theid
and that meets the condition that it has to be associated with this object. -
collection.build(attributes = {})
- returns a new object of the collection type that has been instantiated withattributes
and linked to this object through the join table, but has not yet been saved. -
collection.create(attributes = {})
- returns a new object of the collection type that has been instantiated withattributes
, linked to this object through the join table, and that has already been saved (if it passed the validation).
Example: A Developer class declares has_and_belongs_to_many :projects
, which will add:
-
Developer#projects
-
Developer#projects<<
-
Developer#projects.delete
-
Developer#projects=
-
Developer#project_ids
-
Developer#project_ids=
-
Developer#projects.clear
-
Developer#projects.empty?
-
Developer#projects.size
-
Developer#projects.find(id)
-
Developer#projects.build
(similar toProject.new("project_id" => id)
) -
Developer#projects.create
(similar toc = Project.new("project_id" => id); c.save; c
)
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. Sohas_and_belongs_to_many :projects
will by default be linked to theProject
class, but if the real class name isSuperProject
, 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, thetable_name
method MUST be declared underneath anyhas_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 aPerson
class that makes ahas_and_belongs_to_many
association will useperson_id
as the defaultforeign_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 isProject
, thehas_and_belongs_to_many
association will useproject_id
as the default associationforeign_key
. -
:conditions
- specify the conditions that the associated object must meet in order to be included as aWHERE
SQL fragment, such asauthorized = 1
. -
:order
- specify the order in which the associated objects are returned as anORDER BY
SQL fragment, such aslast_name, first_name DESC
-
:uniq
- if set totrue
, duplicate associated objects will be ignored by accessors and query methods -
:finder_sql
- overwrite the default generated SQL statement used to fetch the association with a manual statement -
:delete_sql
- overwrite the default generated SQL statement used to remove links between the associated classes with a manual statement -
:insert_sql
- overwrite the default generated SQL statement used to add links between the associated classes with a manual statement -
: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 theGROUP 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 inSELECT * FROM
, but can be changed if, for example, you 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 'lib/active_record/associations.rb', line 971 def has_and_belongs_to_many(association_id, = {}, &extension) reflection = create_has_and_belongs_to_many_reflection(association_id, , &extension) add_multiple_associated_save_callbacks(reflection.name) collection_accessor_methods(reflection, HasAndBelongsToManyAssociation) # Don't use a before_destroy callback since users' before_destroy # callbacks will be executed after the association is wiped out. old_method = "destroy_without_habtm_shim_for_#{reflection.name}" class_eval <<-end_eval unless method_defined?(old_method) alias_method :#{old_method}, :destroy_without_callbacks def destroy_without_callbacks #{reflection.name}.clear #{old_method} end end_eval add_association_callbacks(reflection.name, ) end |
#has_many(association_id, options = {}, &extension) ⇒ Object
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 asbelongs_to
and dependent on this model. -
collection=objects
- replaces the collections content by deleting and adding objects as appropriate. -
collection_singular_ids
- returns an array of the associated objects’ ids -
collection_singular_ids=ids
- replace the collection with the objects identified by the primary keys inids
-
collection.clear
- removes every object from the collection. This destroys the associated objects if they are associated with:dependent => :destroy
, deletes them directly from the database if:dependent => :delete_all
, otherwise sets their foreign keys to NULL. -
collection.empty?
- returnstrue
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 one or more new objects of the collection type that have been instantiated withattributes
and linked to this object through a foreign key, but have not yet been saved. Note: This only works if an associated object already exists, not if it’snil
! -
collection.create(attributes = {})
- returns a new object of the collection type that has been instantiated withattributes
, 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’snil
!
Example: A Firm
class declares has_many :clients
, which will add:
-
Firm#clients
(similar toClients.find :all, :conditions => "firm_id = #{id}"
) -
Firm#clients<<
-
Firm#clients.delete
-
Firm#clients=
-
Firm#client_ids
-
Firm#client_ids=
-
Firm#clients.clear
-
Firm#clients.empty?
(similar tofirm.clients.size == 0
) -
Firm#clients.size
(similar toClient.count "firm_id = #{id}"
) -
Firm#clients.find
(similar toClient.find(id, :conditions => "firm_id = #{id}")
) -
Firm#clients.build
(similar toClient.new("firm_id" => id)
) -
Firm#clients.create
(similar toc = 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. Sohas_many :products
will by default be linked to theProduct
class, but if the real class name isSpecialProduct
, 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 aWHERE
SQL fragment, such asprice > 5 AND name LIKE 'B%'
. -
:order
- specify the order in which the associated objects are returned as anORDER BY
SQL fragment, such aslast_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 aPerson
class that makes ahas_many
association will useperson_id
as the defaultforeign_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 toNULL
without calling their save callbacks. -
: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 not:counter_sql
,:counter_sql
will be generated by replacingSELECT ... FROM
withSELECT 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 theGROUP 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 inSELECT * 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 through which to perform the query. Options for:class_name
and:foreign_key
are ignored, as the association uses the source reflection. You can only use a:through
query through abelongs_to
orhas_many
association on the join model. -
:source
: Specifies the source association name used byhas_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
onSubscription
, unless a:source
is given. -
:source_type
: Specifies type of the source association used byhas_many :through
queries where the source association is a polymorphicbelongs_to
. -
:uniq
- if set totrue
, duplicates will be omitted from the collection. Useful in conjunction with:through
.
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 'lib/active_record/associations.rb', line 682 def has_many(association_id, = {}, &extension) reflection = create_has_many_reflection(association_id, , &extension) configure_dependency_for_has_many(reflection) if [:through] collection_accessor_methods(reflection, HasManyThroughAssociation, false) else add_multiple_associated_save_callbacks(reflection.name) add_association_callbacks(reflection.name, reflection.) collection_accessor_methods(reflection, HasManyAssociation) end end |
#has_one(association_id, options = {}) ⇒ Object
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
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# File 'lib/active_record/associations.rb', line 741 def has_one(association_id, = {}) reflection = create_has_one_reflection(association_id, ) ivar = "@#{reflection.name}" module_eval do after_save <<-EOF association = instance_variable_get("#{ivar}") if instance_variable_defined?("#{ivar}") if !association.nil? && (new_record? || association.new_record? || association["#{reflection.primary_key_name}"] != id) association["#{reflection.primary_key_name}"] = id association.save(true) end EOF end association_accessor_methods(reflection, HasOneAssociation) association_constructor_method(:build, reflection, HasOneAssociation) association_constructor_method(:create, reflection, HasOneAssociation) configure_dependency_for_has_one(reflection) end |