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.
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)
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
| | | has_many generated methods | habtm | has_many | :through ----------------------------------+-------+----------+---------- others | X | X | X others=(other,other,...) | X | X | X other_ids | X | X | X other_ids=(id,id,...) | X | 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 | X others.create!(attributes={}) | X | X | X others.size | X | X | X others.length | X | X | X others.count | X | X | X others.sum(args*,&block) | X | X | X others.empty? | X | X | X others.clear | X | 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.exists? | X | X | X others.uniq | X | X | X others.reset | X | X | X
Active Record associations can be used to describe one-to-one, one-to-many
and many-to-many relationships between models. Each model uses an
association to describe its role in the relation. The
belongs_to
association is always 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.
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.
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.
You can set the :autosave option on a has_one
,
belongs_to
, has_many
, or
has_and_belongs_to_many
association. Setting it to
true
will always save the members, whereas setting it
to false
will never save the members. More details
about :autosave option is available at autosave_association.rb .
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.
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.
Similar to the normal callbacks that hook into the life cycle 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.
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.
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.
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.
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 for
belongs_to
and has_one
, or the collection of
associated objects for has_many
and
has_and_belongs_to_many
.
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
Similarly you can go through a has_one
association on the join
model:
class Group < ActiveRecord::Base has_many :users has_many :avatars, :through => :users end class User < ActiveRecord::Base belongs_to :group has_one :avatar end class Avatar < ActiveRecord::Base belongs_to :user end @group = Group.first @group.users.collect { |u| u.avatar }.flatten # select all avatars for all users in the group @group.avatars # selects all avatars by going through the User join model.
An important caveat with going through has_one
or
has_many
associations on the join model is that these
associations are read-only. For example, the following
would not work following the previous example:
@group.avatars << Avatar.new # this would work if User belonged_to Avatar rather than the other way around @group.avatars.delete(@group.avatars.last) # so would this
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.
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. 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 2.
class Post < ActiveRecord::Base belongs_to :author has_many :comments end
Consider the following loop using the class above:
for post in Post.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. After loading the posts, find
will collect the author_id
from each one and load all the
referenced authors with one query. Doing so will cut down the number of
queries from 201 to 102.
We can improve upon the situation further by referencing both associations in the finder with:
for post in Post.find(:all, :include => [ :author, :comments ])
This will load all comments with a single query. This reduces the total
number of queries to 3. More generally the number of queries will be 1 plus
the number of associations named (except if some of the associations are
polymorphic belongs_to
- see below).
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 only one table is loaded at a time, conditions or orders cannot reference tables other than the main one. If this is the case Active Record falls back to the previously used LEFT OUTER JOIN based strategy. For example
Post.includes([:author, :comments]).where(['comments.approved = ?', true]).all
This will result in a single SQL query with joins along the lines of:
LEFT OUTER JOIN comments ON comments.post_id = posts.id
and
LEFT OUTER JOIN authors ON authors.id = posts.author_id
. Note
that using conditions like this can have unintended consequences. In the
above example posts with no approved comments are not returned at all,
because the conditions apply to the SQL statement as a whole and not just
to the association. You must disambiguate column references for this
fallback to happen, for example :order => "author.name
DESC"
will work but :order => "name
DESC"
will not.
If you do want eager load only some members of an association it is usually
more natural to :include
an association which has conditions
defined on it:
class Post < ActiveRecord::Base has_many :approved_comments, :class_name => 'Comment', :conditions => ['approved = ?', true] end Post.find(:all, :include => :approved_comments)
This will load posts and eager load the approved_comments
association, which contains only those comments that have been approved.
If you eager load an association with a specified :limit
option, it will be ignored, returning all the associated objects:
class Picture < ActiveRecord::Base has_many :most_recent_comments, :class_name => 'Comment', :order => 'id DESC', :limit => 10 end Picture.find(:first, :include => :most_recent_comments).most_recent_comments # => returns all associated comments.
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 supported with polymorphic associations.
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)
This will execute one query to load the addresses and load the addressables
with one query per addressable type. For example if all the addressables
are either of class Person or Company then a total of 3 queries will be
executed. The list of addressable types to load is determined on the back
of the addresses loaded. This is not supported if Active Record has to
fallback to the previous implementation of eager loading and 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 query.
Active Record 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, :joins => :comments # => SELECT ... FROM posts INNER JOIN comments ON ... Post.find :all, :joins => :special_comments # STI # => SELECT ... FROM posts INNER JOIN comments ON ... AND comments.type = 'SpecialComment' Post.find :all, :joins => [:comments, :special_comments] # special_comments is the reflection name, posts is the parent table name # => SELECT ... FROM posts INNER JOIN comments ON ... INNER JOIN comments special_comments_posts
Acts as tree example:
TreeMixin.find :all, :joins => :children # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ... TreeMixin.find :all, :joins => {:children => :parent} # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ... INNER JOIN parents_mixins ... TreeMixin.find :all, :joins => {:children => {:parent => :children}} # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ... INNER JOIN parents_mixins ... INNER JOIN mixins childrens_mixins_2
Has and Belongs to Many join tables use the same idea, but add a
_join
suffix:
Post.find :all, :joins => :categories # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ... Post.find :all, :joins => {:categories => :posts} # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ... INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories Post.find :all, :joins => {:categories => {:posts => :categories}} # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ... INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories INNER JOIN categories_posts categories_posts_join INNER JOIN categories categories_posts_2
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, :joins => :comments, :joins => "inner join comments ..." # => SELECT ... FROM posts INNER JOIN comments_posts ON ... INNER JOIN comments ... Post.find :all, :joins => [:comments, :special_comments], :joins => "inner join comments ..." # => SELECT ... FROM posts INNER JOIN comments comments_posts ON ... INNER 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 Client < ActiveRecord::Base; end end end
When Firm#clients
is called, it will in turn call
MyApplication::Business::Client.find_all_by_firm_id(firm.id)
.
If you want to associate with a class in another module scope, this can be
done by specifying the complete class name.
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
When you specify an association there is usually an association on the associated model that specifies the same relationship in reverse. For example, with the following models:
class Dungeon < ActiveRecord::Base has_many :traps has_one :evil_wizard end class Trap < ActiveRecord::Base belongs_to :dungeon end class EvilWizard < ActiveRecord::Base belongs_to :dungeon end
The traps
association on Dungeon
and the the
dungeon
association on Trap
are the inverse of
each other and the inverse of the dungeon
association on
EvilWizard
is the evil_wizard
association on
Dungeon
(and vice-versa). By default, Active Record doesn't
know anything about these inverse relationships and so no object loading
optimisation is possible. For example:
d = Dungeon.first t = d.traps.first d.level == t.dungeon.level # => true d.level = 10 d.level == t.dungeon.level # => false
The Dungeon
instances d
and
t.dungeon
in the above example refer to the same object data
from the database, but are actually different in-memory copies of that
data. Specifying the :inverse_of
option on associations lets
you tell Active Record about inverse relationships and it will optimise
object loading. For example, if we changed our model definitions to:
class Dungeon < ActiveRecord::Base has_many :traps, :inverse_of => :dungeon has_one :evil_wizard, :inverse_of => :dungeon end class Trap < ActiveRecord::Base belongs_to :dungeon, :inverse_of => :traps end class EvilWizard < ActiveRecord::Base belongs_to :dungeon, :inverse_of => :evil_wizard end
Then, from our code snippet above, d
and
t.dungeon
are actually the same in-memory instance and our
final d.level == t.dungeon.level
will return
true
.
There are limitations to :inverse_of
support:
does not work with :through
associations.
does not work with :polymorphic
associations.
for belongs_to
associations has_many
inverse
associations are ignored.
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
.
All of the association macros can be specialized through options. This makes cases more complex than the simple and guessable ones possible.
Specifies a one-to-one association with another class. This method should
only be used if this class contains the foreign key. If the other class
contains the foreign key, then you should use has_one
instead.
See also ActiveRecord::Associations::ClassMethods's overview on when to use
has_one
and when to use belongs_to
.
Methods will be added for retrieval and query for a single associated object, for which this object holds an id:
Returns the associated object. nil
is returned if none is
found.
Assigns the associate object, extracts the primary key, and sets it as the foreign key.
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.
Returns a new object of the associated type that has been instantiated with
attributes
, linked to this object through a foreign key, and
that has already been saved (if it passed the validation).
(association
is replaced with the symbol passed as the first
argument, so belongs_to :author
would add among others
author.nil?
.)
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#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.
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.
Specify the conditions that the associated object must meet in order to be
included as a WHERE
SQL fragment, such as authorized =
1
.
By default, this is *
as in SELECT * FROM
, but
can be changed if, for example, you want to do a join but not include the
joined columns. Do not forget to include the primary and foreign keys,
otherwise it will raise an error.
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 use
"person_id" as the default :foreign_key
. Similarly,
belongs_to :favorite_person, :class_name =>
"Person"
will use a foreign key of "favorite_person_id".
Specify the method that returns the primary key of associated object used for the association. By default this is id.
If set to :destroy
, the associated object is destroyed when
this object is. If set to :delete
, the associated object is
deleted without calling its destroy method. This option
should not be specified when belongs_to
is used in conjunction
with a has_many
relationship on another class because of the
potential to leave orphaned records behind.
Caches the number of belonging objects on the associate class through the
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 providing a
column name instead of a true
/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
.
Specify second-order associations that should be eager loaded when this object is loaded.
Specify this association is a polymorphic association by passing
true
. 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
).
If true, the associated object is readonly through the association.
If false
, don't validate the associated objects when saving
the parent object. false
by default.
If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. If false, never save or destroy the associated object. By default, only save the associated object if it's a new record.
If true, the associated object will be touched (the updated_at/on attributes set to now) when this record is either saved or destroyed. If you specify a symbol, that attribute will be updated with the current time instead of the updated_at/on attribute.
Specifies the name of the has_one
or has_many
association on the associated object that is the inverse of this
belongs_to
association. Does not work in combination with the
:polymorphic
options. See
ActiveRecord::Associations::ClassMethods's overview on Bi-directional
associations for more detail.
Option examples:
belongs_to :firm, :foreign_key => "client_of" belongs_to :person, :primary_key => "name", :foreign_key => "person_name" 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 belongs_to :project, :readonly => true belongs_to :post, :counter_cache => true belongs_to :company, :touch => true belongs_to :company, :touch => :employees_last_updated_at
# File lib/active_record/associations.rb, line 1219 def belongs_to(association_id, options = {}) reflection = create_belongs_to_reflection(association_id, options) if reflection.options[:polymorphic] association_accessor_methods(reflection, BelongsToPolymorphicAssociation) else association_accessor_methods(reflection, BelongsToAssociation) association_constructor_method(:build, reflection, BelongsToAssociation) association_constructor_method(:create, reflection, BelongsToAssociation) end add_counter_cache_callbacks(reflection) if options[:counter_cache] add_touch_callbacks(reflection, options[:touch]) if options[:touch] configure_dependency_for_belongs_to(reflection) end
Specifies a many-to-many relationship with another class. This 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.
The join table should not have a primary key or a model associated with it. You must manually generate the join table with a migration such as this:
class CreateDevelopersProjectsJoinTable < ActiveRecord::Migration def self.up create_table :developers_projects, :id => false do |t| t.integer :developer_id t.integer :project_id end end def self.down drop_table :developers_projects end end
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:
Returns an array of all the associated objects. An empty array is returned if none are found.
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). Note that this operation instantly fires
update sql without waiting for the save or update call on the parent
object.
Removes one or more objects from the collection by removing their associations from the join table. This does not destroy the objects.
Replaces the collection's content by deleting and adding objects as appropriate.
Returns an array of the associated objects' ids.
Replace the collection by the objects identified by the primary keys in
ids
.
Removes every object from the collection. This does not destroy the objects.
Returns true
if there are no associated objects.
Returns the number of associated objects.
Finds an associated object responding to the id
and that meets
the condition that it has to be associated with this object. Uses the same
rules as ActiveRecord::Base.find.
Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::Base.exists?.
Returns a new object of the collection type that has been instantiated with
attributes
and linked to this object through the join table,
but has not yet been saved.
Returns a new object of the collection type that has been instantiated with
attributes
, linked to this object through the join table, and
that has already been saved (if it passed the validation).
(collection
is replaced with the symbol passed as the first
argument, so has_and_belongs_to_many :categories
would add
among others categories.empty?
.)
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.exists?(...)
Developer#projects.build
(similar to
Project.new("project_id" => id)
)
Developer#projects.create
(similar to c =
Project.new("project_id" => id); c.save; c
)
The declaration may include an options hash to specialize the behavior of the association.
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.
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.
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
to Project will use "person_id" as the default :foreign_key
.
Specify the foreign key used for the association on the receiving side of
the association. By default this is guessed to be the name of the
associated class in lower-case and "_id" suffixed. So if a Person class
makes a has_and_belongs_to_many
association to Project, the
association will use "project_id" as the default
:association_foreign_key
.
Specify the conditions that the associated object must meet in order to be
included as a WHERE
SQL fragment, such as authorized =
1
. Record creations from the association are scoped if a hash is
used. has_many :posts, :conditions => {:published =>
true}
will create published posts with
@blog.posts.create
or @blog.posts.build
.
Specify the order in which the associated objects are returned as an
ORDER BY
SQL fragment, such as last_name, first_name
DESC
If true, duplicate associated objects will be ignored by accessors and query methods.
Overwrite the default generated SQL statement used to fetch the association with a manual statement
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 replacing SELECT ...
FROM
with SELECT COUNT(*) FROM
.
Overwrite the default generated SQL statement used to remove links between the associated classes with a manual statement.
Overwrite the default generated SQL statement used to add links between the associated classes with a manual statement.
Anonymous module for extending the proxy, see "Association extensions".
Specify second-order associations that should be eager loaded when the collection is loaded.
An attribute name by which the result should be grouped. Uses the
GROUP BY
SQL-clause.
Combined with :group
this can be used to filter the records
that a GROUP BY
returns. Uses the HAVING
SQL-clause.
An integer determining the limit on the number of rows that should be returned.
An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
By default, this is *
as in SELECT * FROM
, but
can be changed if, for example, you want to do a join but not include the
joined columns. Do not forget to include the primary and foreign keys,
otherwise it will raise an error.
If true, all the associated objects are readonly through the association.
If false
, don't validate the associated objects when saving
the parent object. true
by default.
If true, always save the associated objects or destroy them if marked for destruction, when saving the parent object. If false, never save or destroy the associated objects. By default, only save associated objects that are new records.
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 :categories, :readonly => true 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 lib/active_record/associations.rb, line 1410 def has_and_belongs_to_many(association_id, options = {}, &extension) reflection = create_has_and_belongs_to_many_reflection(association_id, options, &extension) collection_accessor_methods(reflection, HasAndBelongsToManyAssociation) configure_destroy_hook_for_has_and_belongs_to_many(reflection) add_association_callbacks(reflection.name, options) end
Specifies a one-to-many association. The following methods for retrieval and query of collections of associated objects will be added:
Returns an array of all the associated objects. An empty array is returned if none are found.
Adds one or more objects to the collection by setting their foreign keys to the collection's primary key. Note that this operation instantly fires update sql without waiting for the save or update call on the parent object.
Removes one or more objects from the collection by setting their foreign
keys to NULL
. Objects will be in addition destroyed if they're
associated with :dependent => :destroy
, and deleted if
they're associated with :dependent => :delete_all
.
Replaces the collections content by deleting and adding objects as
appropriate. If the :through
option is true callbacks in the
join models are triggered except destroy callbacks, since deletion is
direct.
Returns an array of the associated objects' ids
Replace the collection with the objects identified by the primary keys in
ids
. This method loads the models and calls
collection=
. See above.
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
.
If the :through
option is true no destroy callbacks are
invoked on the join models. Join models are directly deleted.
Returns true
if there are no associated objects.
Returns the number of associated objects.
Finds an associated object according to the same rules as ActiveRecord::Base.find.
Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::Base.exists?.
Returns one or more new objects of the collection type that have been
instantiated with attributes
and linked to this object through
a foreign key, but have not yet been saved.
Returns a new object of the collection type that has been instantiated with
attributes
, linked to this object through a foreign key, and
that has already been saved (if it passed the validation).
Note: This only works if the base model already exists in
the DB, not if it is a new (unsaved) record!
(Note: 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:
Firm#clients
(similar to Clients.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 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.exists?(:name => 'ACME')
(similar to
Client.exists?(:name => 'ACME', :firm_id => firm.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.
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.
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%'
. Record creations from the association are
scoped if a hash is used. has_many :posts, :conditions =>
{:published => true}
will create published posts with
@blog.posts.create
or @blog.posts.build
.
Specify the order in which the associated objects are returned as an
ORDER BY
SQL fragment, such as last_name, first_name
DESC
.
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
.
Specify the method that returns the primary key used for the association.
By default this is id
.
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. If set to :restrict
this object cannot be deleted
if it has any associated object.
Warning: This option is ignored when used with
:through
option.
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.
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 replacing SELECT ...
FROM
with SELECT COUNT(*) FROM
.
Specify a named module for extending the proxy. See "Association extensions".
Specify second-order associations that should be eager loaded when the collection is loaded.
An attribute name by which the result should be grouped. Uses the
GROUP BY
SQL-clause.
Combined with :group
this can be used to filter the records
that a GROUP BY
returns. Uses the HAVING
SQL-clause.
An integer determining the limit on the number of rows that should be returned.
An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
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. Do not forget to include the primary and foreign keys,
otherwise it will raise an error.
Specifies a polymorphic interface (See belongs_to
).
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 a belongs_to
,
has_one
or has_many
association on the join
model. The collection of join models can be managed via the collection API.
For example, new join models are created for newly associated objects, and
if some are gone their rows are deleted (directly, no destroy callbacks are
triggered).
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.
Specifies type of the source association used by has_many
:through
queries where the source association is a polymorphic
belongs_to
.
If true, duplicates will be omitted from the collection. Useful in
conjunction with :through
.
If true, all the associated objects are readonly through the association.
If false
, don't validate the associated objects when saving
the parent object. true by default.
If true, always save the associated objects or destroy them if marked for destruction, when saving the parent object. If false, never save or destroy the associated objects. By default, only save associated objects that are new records.
Specifies the name of the belongs_to
association on the
associated object that is the inverse of this has_many
association. Does not work in combination with :through
or
:as
options. See ActiveRecord::Associations::ClassMethods's
overview on Bi-directional associations for more detail.
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 :reports, :readonly => true 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 lib/active_record/associations.rb, line 982 def has_many(association_id, options = {}, &extension) reflection = create_has_many_reflection(association_id, options, &extension) configure_dependency_for_has_many(reflection) add_association_callbacks(reflection.name, reflection.options) if options[:through] collection_accessor_methods(reflection, HasManyThroughAssociation) else collection_accessor_methods(reflection, HasManyAssociation) end end
Specifies a one-to-one association with another class. This method should
only be used if the other class contains the foreign key. If the current
class contains the foreign key, then you should use belongs_to
instead. See also ActiveRecord::Associations::ClassMethods's overview on
when to use #has_one and
when to use belongs_to.
The following methods for retrieval and query of a single associated object will be added:
Returns the associated object. nil
is returned if none is
found.
Assigns the associate object, extracts the primary key, sets it as the foreign key, and saves the associate object.
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
.
Returns a new object of the associated type that has been instantiated with
attributes
, linked to this object through a foreign key, and
that has already been saved (if it passed the validation).
(association
is replaced with the symbol passed as the first
argument, so has_one :manager
would add among others
manager.nil?
.)
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#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:
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.
Specify the conditions that the associated object must meet in order to be
included as a WHERE
SQL fragment, such as rank =
5
. Record creation from the association is scoped if a hash is used.
has_one :account, :conditions => {:enabled => true}
will
create an enabled account with @company.create_account
or
@company.build_account
.
Specify the order in which the associated objects are returned as an
ORDER BY
SQL fragment, such as last_name, first_name
DESC
.
If set to :destroy
, the associated object is destroyed when
this object is. If set to :delete
, the associated object is
deleted without calling its destroy method. If set to
:nullify
, the associated object's foreign key is set to
NULL
. Also, association is assigned.
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
.
Specify the method that returns the primary key used for the association.
By default this is id
.
Specify second-order associations that should be eager loaded when this object is loaded.
Specifies a polymorphic interface (See belongs_to
).
By default, this is *
as in SELECT * FROM
, but
can be changed if, for example, you want to do a join but not include the
joined columns. Do not forget to include the primary and foreign keys,
otherwise it will raise an error.
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 a has_one
or
belongs_to
association on the join model.
Specifies the source association name used by has_one :through
queries. Only use it if the name cannot be inferred from the association.
has_one :favorite, :through => :favorites
will look for a
:favorite
on Favorite, unless a :source
is given.
Specifies type of the source association used by has_one
:through
queries where the source association is a polymorphic
belongs_to
.
If true, the associated object is readonly through the association.
If false
, don't validate the associated object when saving the
parent object. false
by default.
If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. If false, never save or destroy the associated object. By default, only save the associated object if it's a new record.
Specifies the name of the belongs_to
association on the
associated object that is the inverse of this has_one
association. Does not work in combination with :through
or
:as
options. See ActiveRecord::Associations::ClassMethods's
overview on Bi-directional associations for more detail.
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 has_one :boss, :readonly => :true has_one :club, :through => :membership has_one :primary_address, :through => :addressables, :conditions => ["addressable.primary = ?", true], :source => :addressable
# File lib/active_record/associations.rb, line 1100 def has_one(association_id, options = {}) if options[:through] reflection = create_has_one_through_reflection(association_id, options) association_accessor_methods(reflection, ActiveRecord::Associations::HasOneThroughAssociation) else reflection = create_has_one_reflection(association_id, options) association_accessor_methods(reflection, HasOneAssociation) association_constructor_method(:build, reflection, HasOneAssociation) association_constructor_method(:create, reflection, HasOneAssociation) configure_dependency_for_has_one(reflection) end end