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#reload_portfolio
-
Project#project_manager
,Project#project_manager=(project_manager)
,Project#reload_project_manager
-
Project#milestones.empty?
,Project#milestones.size
,Project#milestones
,Project#milestones<<(milestone)
,Project#milestones.delete(milestone)
,Project#milestones.destroy(milestone)
,Project#milestones.find(milestone_id)
,Project#milestones.build
,Project#milestones.create
-
Project#categories.empty?
,Project#categories.size
,Project#categories
,Project#categories<<(category1)
,Project#categories.delete(category1)
,Project#categories.destroy(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, using an association with the same name as one provided by ActiveRecord::Base
will override the method inherited through ActiveRecord::Base
and will break things. For instance, attributes
and connection
would be bad choices for association names, because those names already exist in the list of ActiveRecord::Base
instance methods.
Auto-generated methods
See also Instance Public methods below for more details.
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
create_other!(attributes={}) | X | | X
reload_other | X | 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 | 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) | 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 | X
others.destroy(other,other,...) | X | X | X
others.destroy_all | X | X | X
others.find(*args) | X | X | X
others.exists? | X | X | X
others.distinct | X | X | X
others.reset | X | X | X
others.reload | X | X | X
Overriding generated methods
Association methods are generated in a module included into the model class, making overrides easy. The original generated method can thus be called with super
:
class Car < ActiveRecord::Base
belongs_to :owner
belongs_to :old_owner
def owner=(new_owner)
self.old_owner = self.owner
super
end
end
The association methods module is included immediately after the generated attributes methods module, meaning an association will override the methods for an attribute with the same name.
Cardinality and associations
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.
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.
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 bigint NOT NULL auto_increment,
account_id bigint default NULL,
name varchar default NULL,
PRIMARY KEY (id)
)
CREATE TABLE accounts (
id bigint 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.
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 AutosaveAssociation
.
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 foreign 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), an
ActiveRecord::RecordNotSaved
exception is raised and the assignment is cancelled. -
If you wish to assign an object to a
has_one
association without saving it, use the#build_association
method (documented below). The object being replaced will still be saved to update its foreign key. -
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, thenpush
returnsfalse
. -
If saving fails while replacing the collection (via
association=
), anActiveRecord::RecordNotSaved
exception is raised and the assignment is cancelled. -
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.
Customizing the query
Associations are built from Relation
objects, and you can use the Relation
syntax to customize them. For example, to add a condition:
class Blog < ActiveRecord::Base
has_many :published_posts, -> { where(published: true) }, class_name: 'Post'
end
Inside the -> { ... }
block you can use all of the usual Relation
methods.
Accessing the owner object
Sometimes it is useful to have access to the owner object when building the query. The owner is passed as a parameter to the block. For example, the following association would find all events that occur on the user's birthday:
class User < ActiveRecord::Base
has_many :birthday_events, ->(user) { where(starts_on: user.birthday) }, class_name: 'Event'
end
Note: Joining, eager loading and preloading of these associations is not possible. These operations happen before instance creation and the scope will be called with a nil
argument.
Association callbacks
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
.
If any of the before_add
callbacks throw an exception, the object will not be added to the collection.
Similarly, if any of the before_remove
callbacks throw an exception, the object will not be removed from the collection.
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.
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: first_name, last_name: last_name)
end
end
end
person = Account.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: first_name, last_name: last_name)
end
end
class Account < ActiveRecord::Base
has_many :people, -> { extending FindOrCreateByNameExtension }
end
class Company < ActiveRecord::Base
has_many :people, -> { extending FindOrCreateByNameExtension }
end
Some extensions can only be made to work with knowledge of the association's internals. Extensions can access relevant state using the following methods (where items
is the name of the association):
-
record.association(:items).owner
- Returns the object the association is part of. -
record.association(:items).reflection
- Returns the reflection object that describes the association. -
record.association(:items).target
- Returns the associated object forbelongs_to
andhas_one
, or the collection of associated objects forhas_many
andhas_and_belongs_to_many
.
However, inside the actual extension code, you will not have access to the record
as above. In this case, you can access proxy_association
. For example, record.association(:items)
and record.items.proxy_association
will return the same object, allowing you to make calls like proxy_association.owner
inside association extensions.
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.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.first
@firm.clients.flat_map { |c| c.invoices } # 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 }.compact # 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
Setting Inverses
If you are using a belongs_to
on the join model, it is a good idea to set the :inverse_of
option on the belongs_to
, which will mean that the following example works correctly (where tags
is a has_many
:through
association):
@post = Post.first
@tag = @post.tags.build name: "ruby"
@tag.save
The last line ought to save the through record (a Tagging
). This will only work if the :inverse_of
is set:
class Tagging < ActiveRecord::Base
belongs_to :post
belongs_to :tag, inverse_of: :taggings
end
If you do not set the :inverse_of
record, the association will do its best to match itself up with the correct inverse. Automatic inverse detection only works on has_many
, has_one
, and belongs_to
associations.
:foreign_key
and :through
options on the associations, or a custom scope, will also prevent the association's inverse from being found automatically.
The automatic guessing of the inverse association uses a heuristic based on the name of the class, so it may not work for all associations, especially the ones with non-standard names.
You can turn off the automatic detection of inverse associations by setting the :inverse_of
option to false
like so:
class Tagging < ActiveRecord::Base
belongs_to :tag, inverse_of: false
end
Nested Associations
You can actually specify any association with the :through
option, including an association which has a :through
option itself. For example:
class Author < ActiveRecord::Base
has_many :posts
has_many :comments, through: :posts
has_many :commenters, through: :comments
end
class Post < ActiveRecord::Base
has_many :comments
end
class Comment < ActiveRecord::Base
belongs_to :commenter
end
@author = Author.first
@author.commenters # => People who commented on posts written by the author
An equivalent way of setting up this association this would be:
class Author < ActiveRecord::Base
has_many :posts
has_many :commenters, through: :posts
end
class Post < ActiveRecord::Base
has_many :comments
has_many :commenters, through: :comments
end
class Comment < ActiveRecord::Base
belongs_to :commenter
end
When using a nested association, you will not be able to modify the association because there is not enough information to know what modification to make. For example, if you tried to add a Commenter
in the example above, there would be no way to tell how to set up the intermediate Post
and Comment
objects.
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.
Note: The attachable_type=
method is being called when assigning an attachable
. The class_name
of the attachable
is passed as a String
.
class Asset < ActiveRecord::Base
belongs_to :attachable, polymorphic: true
def attachable_type=(class_name)
super(class_name.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.
project.milestones # fetches milestones from the database
project.milestones.size # uses the milestone cache
project.milestones.empty? # uses the milestone cache
project.milestones.reload.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. It is one of the easiest ways to prevent the dreaded N+1 problem in which fetching 100 posts that each need to display their author triggers 101 database queries. Through the use of eager loading, the number of queries will be reduced from 101 to 2.
class Post < ActiveRecord::Base
belongs_to :author
has_many :comments
end
Consider the following loop using the class above:
Post.all.each do |post|
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:
Post.includes(:author).each do |post|
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 of 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:
Post.includes(:author, :comments).each do |post|
This will load all comments with a single query. This reduces the total number of queries to 3. In general, 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:
Post.includes(:author, { comments: { author: :gravatar } }).each do |post|
The above code will load all the comments and all of their associated authors and gravatars. You can mix and match any combination of symbols, arrays, and hashes to retrieve 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])
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 want to load all posts (including posts with no approved comments), then write your own LEFT OUTER JOIN
query using ON
:
Post.joins("LEFT OUTER JOIN comments ON comments.post_id = posts.id AND comments.approved = '1'")
In this case, it is usually more natural to include an association which has conditions defined on it:
class Post < ActiveRecord::Base
has_many :approved_comments, -> { where(approved: true) }, class_name: 'Comment'
end
Post.includes(: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, -> { order('id DESC').limit(10) }, class_name: 'Comment'
end
Picture.includes(:most_recent_comments).first.most_recent_comments # => returns all associated comments.
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.includes(: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.
Table Aliasing
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.joins(:comments)
# => SELECT ... FROM posts INNER JOIN comments ON ...
Post.joins(:special_comments) # STI
# => SELECT ... FROM posts INNER JOIN comments ON ... AND comments.type = 'SpecialComment'
Post.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.joins(:children)
# => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
TreeMixin.joins(children: :parent)
# => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
INNER JOIN parents_mixins ...
TreeMixin.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.joins(:categories)
# => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
Post.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.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 ActiveRecord::QueryMethods#joins
method, those table names will take precedence over the eager associations:
Post.joins(:comments).joins("inner join comments ...")
# => SELECT ... FROM posts INNER JOIN comments_posts ON ... INNER JOIN comments ...
Post.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.
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 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
Bi-directional associations
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 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 can guess the inverse of the association based on the name of the class. The result is the following:
d = Dungeon.first
t = d.traps.first
d.object_id == t.dungeon.object_id # => true
The Dungeon
instances d
and t.dungeon
in the above example refer to the same in-memory instance since the association matches the name of the class. The result would be the same if we added :inverse_of
to our model definitions:
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
For more information, see the documentation for the :inverse_of
option.
Deleting from associations
Dependent associations
has_many
, has_one
, and belongs_to
associations support the :dependent
option. This allows you to specify that associated records should be deleted when the owner is deleted.
For example:
class Author
has_many :posts, dependent: :destroy
end
Author.find(1).destroy # => Will destroy all of the author's posts, too
The :dependent
option can have different values which specify how the deletion is done. For more information, see the documentation for this option on the different specific association types. When no option is given, the behavior is to do nothing with the associated records when destroying a record.
Note that :dependent
is implemented using Rails' callback system, which works by processing callbacks in order. Therefore, other callbacks declared either before or after the :dependent
option can affect what it does.
Note that :dependent
option is ignored for has_one
:through
associations.
Delete or destroy?
has_many
and has_and_belongs_to_many
associations have the methods destroy
, delete
, destroy_all
and delete_all
.
For has_and_belongs_to_many
, delete
and destroy
are the same: they cause the records in the join table to be removed.
For has_many
, destroy
and destroy_all
will always call the destroy
method of the record(s) being removed so that callbacks are run. However delete
and delete_all
will either do the deletion according to the strategy specified by the :dependent
option, or if no :dependent
option is given, then it will follow the default strategy. The default strategy is to do nothing (leave the foreign keys with the parent ids set), except for has_many
:through
, where the default strategy is delete_all
(delete the join records, without running their callbacks).
There is also a clear
method which is the same as delete_all
, except that it returns the association rather than the records which have been deleted.
What gets deleted?
There is a potential pitfall here: has_and_belongs_to_many
and has_many
:through
associations have records in join tables, as well as the associated records. So when we call one of these deletion methods, what exactly should be deleted?
The answer is that it is assumed that deletion on an association is about removing the link between the owner and the associated object(s), rather than necessarily the associated objects themselves. So with has_and_belongs_to_many
and has_many
:through
, the join records will be deleted, but the associated records won't.
This makes sense if you think about it: if you were to call post.tags.delete(Tag.find_by(name: 'food'))
you would want the 'food' tag to be unlinked from the post, rather than for the tag itself to be removed from the database.
However, there are examples where this strategy doesn't make sense. For example, suppose a person has many projects, and each project has many tasks. If we deleted one of a person's tasks, we would probably not want the project to be deleted. In this scenario, the delete method won't actually work: it can only be used if the association on the join model is a belongs_to
. In other situations you are expected to perform operations directly on either the associated records or the :through
association.
With a regular has_many
there is no distinction between the “associated records” and the “link”, so there is only one choice for what gets deleted.
With has_and_belongs_to_many
and has_many
:through
, if you want to delete the associated records themselves, you can always do something along the lines of person.tasks.each(&:destroy)
.
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.
Instance Public methods
belongs_to(name, scope = nil, **options) Link
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:
association
is a placeholder for the symbol passed as the name
argument, so belongs_to :author
would add among others author.nil?
.
- association
-
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. No modification or deletion of existing records takes place.
- build_association(attributes = {})
-
Returns a new object of the associated type that has been instantiated with
attributes
and linked to this object through a foreign key, but has not yet been saved. - create_association(attributes = {})
-
Returns a new object of the associated type that has been instantiated with
attributes
, linked to this object through a foreign key, and that has already been saved (if it passed the validation). - create_association!(attributes = {})
-
Does the same as
create_association
, but raisesActiveRecord::RecordInvalid
if the record is invalid. - reload_association
-
Returns the associated object, forcing a database read.
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#build_author
(similar topost.author = Author.new
) -
Post#create_author
(similar topost.author = Author.new; post.author.save; post.author
) -
Post#create_author!
(similar topost.author = Author.new; post.author.save!; post.author
) -
Post#reload_author
The declaration can also include an options
hash to specialize the behavior of the association.
Scopes
You can pass a second argument scope
as a callable (i.e. proc or lambda) to retrieve a specific record or customize the generated query when you access the associated object.
Scope examples:
belongs_to :firm, -> { where(id: 2) }
belongs_to :user, -> { joins(:friends) }
belongs_to :level, ->(game) { where("game_level > ?", game.current_level) }
Options
- :class_name
-
Specify the class name of the association. Use it only if that name can't be inferred from the association name. So
belongs_to :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. - :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 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”.If you are going to modify the association (rather than just read from it), then it is a good idea to set the
:inverse_of
option. - :foreign_type
-
Specify the column used to store the associated object's type, if this is a polymorphic association. By default this is guessed to be the name of the association with a “_type” suffix. So a class that defines a
belongs_to :taggable, polymorphic: true
association will use “taggable_type” as the default:foreign_type
. - :primary_key
-
Specify the method that returns the primary key of associated object used for the association. By default this is
id
. - :dependent
-
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:destroy_async
, the associated object is scheduled to be destroyed in a background job. This option should not be specified whenbelongs_to
is used in conjunction with ahas_many
relationship on another class because of the potential to leave orphaned records behind. - :counter_cache
-
Caches the number of belonging objects on the associate class through the use of
CounterCache::ClassMethods#increment_counter
andCounterCache::ClassMethods#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 ascomments_count
for a belonging Comment class) is used on the associate class (such as a Post class) - that is the migration for#{table_name}_count
is created on the associate class (such thatPost.comments_count
will return the count cached, see note below). 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 usingattr_readonly
. - :polymorphic
-
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 theattr_readonly
list in the associated classes (e.g.class Post; attr_readonly :comments_count; end
). - :validate
-
When set to
true
, validates new objects added to association when saving the parent object.false
by default. If you want to ensure associated objects are revalidated on every update, usevalidates_associated
. - :autosave
-
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.
Note that
NestedAttributes::ClassMethods#accepts_nested_attributes_for
sets:autosave
totrue
. - :touch
-
If true, the associated object will be touched (the updated_at/on attributes set to current time) when this record is either saved or destroyed. If you specify a symbol, that attribute will be updated with the current time in addition to the updated_at/on attribute. Please note that with touching no validation is performed and only the
after_touch
,after_commit
andafter_rollback
callbacks are executed. - :inverse_of
-
Specifies the name of the
has_one
orhas_many
association on the associated object that is the inverse of thisbelongs_to
association. See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail. - :optional
-
When set to
true
, the association will not have its presence validated. - :required
-
When set to
true
, the association will also have its presence validated. This will validate the association itself, not the id. You can use:inverse_of
to avoid an extra query during validation. NOTE:required
is set totrue
by default and is deprecated. If you don't want to have association presence validated, useoptional: true
. - :default
-
Provide a callable (i.e. proc or lambda) to specify that the association should be initialized with a particular record before validation.
- :strict_loading
-
Enforces strict loading every time the associated record is loaded through this association.
- :ensuring_owner_was
-
Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.
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, ->(o) { where "discounts > ?", o.payments_count },
class_name: "Coupon", foreign_key: "coupon_id"
belongs_to :attachable, polymorphic: true
belongs_to :project, -> { readonly }
belongs_to :post, counter_cache: true
belongs_to :comment, touch: true
belongs_to :company, touch: :employees_last_updated_at
belongs_to :user, optional: true
belongs_to :account, default: -> { company.account }
belongs_to :account, strict_loading: true
has_and_belongs_to_many(name, scope = nil, **options, &extension) Link
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” precedes “P” alphabetically. 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. If your tables share a common prefix, it will only appear once at the beginning. For example, the tables “catalog_categories” and “catalog_products” generate a join table name of “catalog_categories_products”.
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[6.0]
def change
create_join_table :developers, :projects
end
end
It's also a good idea to add indexes to each of those columns to speed up the joins process. However, in MySQL it is advised to add a compound index for both of the columns as MySQL only uses one index per table during the lookup.
Adds the following methods for retrieval and query:
collection
is a placeholder for the symbol passed as the name
argument, so has_and_belongs_to_many :categories
would add among others categories.empty?
.
- collection
-
Returns a
Relation
of all the associated objects. An emptyRelation
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). Note that this operation instantly fires update SQL without waiting for the save or update call on the parent object, unless the parent object is a new record. - 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.destroy(object, …)
-
Removes one or more objects from the collection by running destroy on each association in the join table, overriding any dependent option. 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 in
ids
. - collection.clear
-
Removes every object from the collection. This does not destroy the objects.
- collection.empty?
-
Returns
true
if there are no associated objects. - collection.size
-
Returns the number of associated objects.
- collection.find(id)
-
Finds an associated object responding to the
id
and that meets the condition that it has to be associated with this object. Uses the same rules asActiveRecord::FinderMethods#find
. - collection.exists?(…)
-
Checks whether an associated object with the given conditions exists. Uses the same rules as
ActiveRecord::FinderMethods#exists?
. - collection.build(attributes = {})
-
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. - collection.create(attributes = {})
-
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.reload
-
Returns a
Relation
of all of the associated objects, forcing a database read. An emptyRelation
is returned if none are found.
Example
A Developer class declares has_and_belongs_to_many :projects
, which will add:
-
Developer#projects
-
Developer#projects<<
-
Developer#projects.delete
-
Developer#projects.destroy
-
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 toProject.new(developer_id: id)
) -
Developer#projects.create
(similar toc = Project.new(developer_id: id); c.save; c
) -
Developer#projects.reload
The declaration may include an options
hash to specialize the behavior of the association.
Scopes
You can pass a second argument scope
as a callable (i.e. proc or lambda) to retrieve a specific set of records or customize the generated query when you access the associated collection.
Scope examples:
has_and_belongs_to_many :projects, -> { includes(:milestones, :manager) }
has_and_belongs_to_many :categories, ->(post) {
where("default_category = ?", post.default_category)
}
Extensions
The extension
argument allows you to pass a block into a has_and_belongs_to_many
association. This is useful for adding new finders, creators and other factory-type methods to be used as part of the association.
Extension examples:
has_and_belongs_to_many :contractors do
def find_or_create_by_name(name)
first_name, last_name = name.split(" ", 2)
find_or_create_by(first_name: first_name, last_name: last_name)
end
end
Options
- :class_name
-
Specify the class name of the association. Use it only if that name can't be inferred from the association name. So
has_and_belongs_to_many :projects
will by default be linked to the Project class, but if the real class name is SuperProject, you'll have to specify it with this option. - :join_table
-
Specify the name of the join table if the default based on lexical order isn't what you want. WARNING: If you're overwriting the table name of either class, the
table_name
method MUST be declared underneath 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 a Person class that makes a
has_and_belongs_to_many
association to Project will use “person_id” as the default:foreign_key
.If you are going to modify the association (rather than just read from it), then it is a good idea to set the
:inverse_of
option. - :association_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
. - :validate
-
When set to
true
, validates new objects added to association when saving the parent object.true
by default. If you want to ensure associated objects are revalidated on every update, usevalidates_associated
. - :autosave
-
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.
Note that
NestedAttributes::ClassMethods#accepts_nested_attributes_for
sets:autosave
totrue
. - :strict_loading
-
Enforces strict loading every time an associated record is loaded through this association.
Option examples:
has_and_belongs_to_many :projects
has_and_belongs_to_many :projects, -> { includes(: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 }
has_and_belongs_to_many :categories, strict_loading: true
# File activerecord/lib/active_record/associations.rb, line 1933 def has_and_belongs_to_many(name, scope = nil, **options, &extension) habtm_reflection = ActiveRecord::Reflection::HasAndBelongsToManyReflection.new(name, scope, options, self) builder = Builder::HasAndBelongsToMany.new name, self, options join_model = builder.through_model const_set join_model.name, join_model private_constant join_model.name middle_reflection = builder.middle_reflection join_model Builder::HasMany.define_callbacks self, middle_reflection Reflection.add_reflection self, middle_reflection.name, middle_reflection middle_reflection.parent_reflection = habtm_reflection include Module.new { class_eval <<-RUBY, __FILE__, __LINE__ + 1 def destroy_associations association(:#{middle_reflection.name}).delete_all(:delete_all) association(:#{name}).reset super end RUBY } hm_options = {} hm_options[:through] = middle_reflection.name hm_options[:source] = join_model.right_reflection.name [:before_add, :after_add, :before_remove, :after_remove, :autosave, :validate, :join_table, :class_name, :extend, :strict_loading].each do |k| hm_options[k] = options[k] if options.key? k end has_many name, scope, **hm_options, &extension _reflections[name.to_s].parent_reflection = habtm_reflection end
has_many(name, scope = nil, **options, &extension) Link
Specifies a one-to-many association. The following methods for retrieval and query of collections of associated objects will be added:
collection
is a placeholder for the symbol passed as the name
argument, so has_many :clients
would add among others clients.empty?
.
- collection
-
Returns a
Relation
of all the associated objects. An emptyRelation
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. Note that this operation instantly fires update SQL without waiting for the save or update call on the parent object, unless the parent object is a new record. This will also run validations and callbacks of associated object(s).
- collection.delete(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 withdependent: :destroy
, and deleted if they're associated withdependent: :delete_all
.If the
:through
option is used, then the join records are deleted (rather than nullified) by default, but you can specifydependent: :destroy
ordependent: :nullify
to override this. - collection.destroy(object, …)
-
Removes one or more objects from the collection by running
destroy
on each record, regardless of any dependent option, ensuring callbacks are run.If the
:through
option is used, then the join records are destroyed instead, not the objects themselves. - collection=objects
-
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 by default. You can specifydependent: :destroy
ordependent: :nullify
to override this. - 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 in
ids
. This method loads the models and callscollection=
. See above. - 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 ifdependent: :delete_all
, otherwise sets their foreign keys toNULL
. If the:through
option is true no destroy callbacks are invoked on the join models. Join models are directly deleted. - collection.empty?
-
Returns
true
if there are no associated objects. - collection.size
-
Returns the number of associated objects.
- collection.find(…)
-
Finds an associated object according to the same rules as
ActiveRecord::FinderMethods#find
. - collection.exists?(…)
-
Checks whether an associated object with the given conditions exists. Uses the same rules as
ActiveRecord::FinderMethods#exists?
. - collection.build(attributes = {}, …)
-
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. - collection.create(attributes = {})
-
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! - collection.create!(attributes = {})
-
Does the same as
collection.create
, but raisesActiveRecord::RecordInvalid
if the record is invalid. - collection.reload
-
Returns a
Relation
of all of the associated objects, forcing a database read. An emptyRelation
is returned if none are found.
Example
A Firm
class declares has_many :clients
, which will add:
-
Firm#clients
(similar toClient.where(firm_id: id)
) -
Firm#clients<<
-
Firm#clients.delete
-
Firm#clients.destroy
-
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.where(firm_id: id).find(id)
) -
Firm#clients.exists?(name: 'ACME')
(similar toClient.exists?(name: 'ACME', firm_id: firm.id)
) -
Firm#clients.build
(similar toClient.new(firm_id: id)
) -
Firm#clients.create
(similar toc = Client.new(firm_id: id); c.save; c
) -
Firm#clients.create!
(similar toc = Client.new(firm_id: id); c.save!
) -
Firm#clients.reload
The declaration can also include an options
hash to specialize the behavior of the association.
Scopes
You can pass a second argument scope
as a callable (i.e. proc or lambda) to retrieve a specific set of records or customize the generated query when you access the associated collection.
Scope examples:
has_many :comments, -> { where(author_id: 1) }
has_many :employees, -> { joins(:address) }
has_many :posts, ->(blog) { where("max_post_length > ?", blog.max_post_length) }
Extensions
The extension
argument allows you to pass a block into a has_many
association. This is useful for adding new finders, creators and other factory-type methods to be used as part of the association.
Extension examples:
has_many :employees do
def find_or_create_by_name(name)
first_name, last_name = name.split(" ", 2)
find_or_create_by(first_name: first_name, last_name: last_name)
end
end
Options
- :class_name
-
Specify the class name of the association. Use it only if that name can't be inferred from the association name. So
has_many :products
will by default be linked to theProduct
class, but if the real class name isSpecialProduct
, you'll have to specify it with this option. - :foreign_key
-
Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a
has_many
association will use “person_id” as the default:foreign_key
.If you are going to modify the association (rather than just read from it), then it is a good idea to set the
:inverse_of
option. - :foreign_type
-
Specify the column used to store the associated object's type, if this is a polymorphic association. By default this is guessed to be the name of the polymorphic association specified on “as” option with a “_type” suffix. So a class that defines a
has_many :tags, as: :taggable
association will use “taggable_type” as the default:foreign_type
. - :primary_key
-
Specify the name of the column to use as the primary key for the association. By default this is
id
. - :dependent
-
Controls what happens to the associated objects when their owner is destroyed. Note that these are implemented as callbacks, and Rails executes callbacks in order. Therefore, other similar callbacks may affect the
:dependent
behavior, and the:dependent
behavior may affect other callbacks.-
nil
do nothing (default). -
:destroy
causes all the associated objects to also be destroyed. -
:destroy_async
destroys all the associated objects in a background job. WARNING: Do not use this option if the association is backed by foreign key constraints in your database. The foreign key constraint actions will occur inside the same transaction that deletes its owner. -
:delete_all
causes all the associated objects to be deleted directly from the database (so callbacks will not be executed). -
:nullify
causes the foreign keys to be set toNULL
. Polymorphic type will also be nullified on polymorphic associations.Callbacks
are not executed. -
:restrict_with_exception
causes anActiveRecord::DeleteRestrictionError
exception to be raised if there are any associated records. -
:restrict_with_error
causes an error to be added to the owner if there are any associated objects.
If using with the
:through
option, the association on the join model must be abelongs_to
, and the records which get deleted are the join records, rather than the associated records.If using
dependent: :destroy
on a scoped association, only the scoped objects are destroyed. For example, if a Post model defineshas_many :comments, -> { where published: true }, dependent: :destroy
anddestroy
is called on a post, only published comments are destroyed. This means that any unpublished comments in the database would still contain a foreign key pointing to the now deleted post. -
- :counter_cache
-
This option can be used to configure a custom named
:counter_cache.
You only need this option, when you customized the name of your:counter_cache
on thebelongs_to
association. - :as
-
Specifies a polymorphic interface (See
belongs_to
). - :through
-
Specifies an association through which to perform the query. This can be any other type of association, including other
:through
associations. Options for:class_name
,:primary_key
and:foreign_key
are ignored, as the association uses the source reflection.If the association on the join model is a
belongs_to
, the collection can be modified and the records on the:through
model will be automatically created and removed as appropriate. Otherwise, the collection is read-only, so you should manipulate the:through
association directly.If you are going to modify the association (rather than just read from it), then it is a good idea to set the
:inverse_of
option on the source association on the join model. This allows associated records to be built which will automatically create the appropriate join model records when they are saved. (See the 'Association Join Models' section above.) - :source
-
Specifies the source association name used by
has_many
:through
queries. Only use it if the name cannot be inferred from the association.has_many :subscribers, through: :subscriptions
will look for either:subscribers
or:subscriber
on Subscription, unless a:source
is given. - :source_type
-
Specifies type of the source association used by
has_many
:through
queries where the source association is a polymorphicbelongs_to
. - :validate
-
When set to
true
, validates new objects added to association when saving the parent object.true
by default. If you want to ensure associated objects are revalidated on every update, usevalidates_associated
. - :autosave
-
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. This option is implemented as a
before_save
callback. Because callbacks are run in the order they are defined, associated objects may need to be explicitly saved in any user-definedbefore_save
callbacks.Note that
NestedAttributes::ClassMethods#accepts_nested_attributes_for
sets:autosave
totrue
. - :inverse_of
-
Specifies the name of the
belongs_to
association on the associated object that is the inverse of thishas_many
association. See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail. - :extend
-
Specifies a module or array of modules that will be extended into the association object returned. Useful for defining methods on associations, especially when they should be shared between multiple association objects.
- :strict_loading
-
Enforces strict loading every time the associated record is loaded through this association.
- :ensuring_owner_was
-
Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.
Option examples:
has_many :comments, -> { order("posted_on") }
has_many :comments, -> { includes(:author) }
has_many :people, -> { where(deleted: false).order("name") }, class_name: "Person"
has_many :tracks, -> { order("position") }, dependent: :destroy
has_many :comments, dependent: :nullify
has_many :tags, as: :taggable
has_many :reports, -> { readonly }
has_many :subscribers, through: :subscriptions, source: :user
has_many :comments, strict_loading: true
has_one(name, scope = nil, **options) Link
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:
association
is a placeholder for the symbol passed as the name
argument, so has_one :manager
would add among others manager.nil?
.
- association
-
Returns the associated object.
nil
is returned if none is found. - association=(associate)
-
Assigns the associate object, extracts the primary key, sets it as the foreign key, and saves the associate object. To avoid database inconsistencies, permanently deletes an existing associated object when assigning a new one, even if the new one isn't saved to database.
- build_association(attributes = {})
-
Returns a new object of the associated type that has been instantiated with
attributes
and linked to this object through a foreign key, but has not yet been saved. - create_association(attributes = {})
-
Returns a new object of the associated type that has been instantiated with
attributes
, linked to this object through a foreign key, and that has already been saved (if it passed the validation). - create_association!(attributes = {})
-
Does the same as
create_association
, but raisesActiveRecord::RecordInvalid
if the record is invalid. - reload_association
-
Returns the associated object, forcing a database read.
Example
An Account class declares has_one :beneficiary
, which will add:
-
Account#beneficiary
(similar toBeneficiary.where(account_id: id).first
) -
Account#beneficiary=(beneficiary)
(similar tobeneficiary.account_id = account.id; beneficiary.save
) -
Account#build_beneficiary
(similar toBeneficiary.new(account_id: id)
) -
Account#create_beneficiary
(similar tob = Beneficiary.new(account_id: id); b.save; b
) -
Account#create_beneficiary!
(similar tob = Beneficiary.new(account_id: id); b.save!; b
) -
Account#reload_beneficiary
Scopes
You can pass a second argument scope
as a callable (i.e. proc or lambda) to retrieve a specific record or customize the generated query when you access the associated object.
Scope examples:
has_one :author, -> { where(comment_id: 1) }
has_one :employer, -> { joins(:company) }
has_one :latest_post, ->(blog) { where("created_at > ?", blog.enabled_at) }
Options
The declaration can also include an options
hash to specialize the behavior of the association.
Options are:
- :class_name
-
Specify the class name of the association. Use it only if that name can't be inferred from the association name. So
has_one :manager
will by default be linked to the Manager class, but if the real class name is Person, you'll have to specify it with this option. - :dependent
-
Controls what happens to the associated object when its owner is destroyed:
-
nil
do nothing (default). -
:destroy
causes the associated object to also be destroyed -
:destroy_async
causes the associated object to be destroyed in a background job. WARNING: Do not use this option if the association is backed by foreign key constraints in your database. The foreign key constraint actions will occur inside the same transaction that deletes its owner. -
:delete
causes the associated object to be deleted directly from the database (so callbacks will not execute) -
:nullify
causes the foreign key to be set toNULL
. Polymorphic type column is also nullified on polymorphic associations.Callbacks
are not executed. -
:restrict_with_exception
causes anActiveRecord::DeleteRestrictionError
exception to be raised if there is an associated record -
:restrict_with_error
causes an error to be added to the owner if there is an associated object
Note that
:dependent
option is ignored when using:through
option. -
- :foreign_key
-
Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a
has_one
association will use “person_id” as the default:foreign_key
.If you are going to modify the association (rather than just read from it), then it is a good idea to set the
:inverse_of
option. - :foreign_type
-
Specify the column used to store the associated object's type, if this is a polymorphic association. By default this is guessed to be the name of the polymorphic association specified on “as” option with a “_type” suffix. So a class that defines a
has_one :tag, as: :taggable
association will use “taggable_type” as the default:foreign_type
. - :primary_key
-
Specify the method that returns the primary key used for the association. By default this is
id
. - :as
-
Specifies a polymorphic interface (See
belongs_to
). - :through
-
Specifies a Join Model through which to perform the query. Options for
:class_name
,:primary_key
, and:foreign_key
are ignored, as the association uses the source reflection. You can only use a:through
query through ahas_one
orbelongs_to
association on the join model.If you are going to modify the association (rather than just read from it), then it is a good idea to set the
:inverse_of
option. - :source
-
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. - :source_type
-
Specifies type of the source association used by
has_one
:through
queries where the source association is a polymorphicbelongs_to
. - :validate
-
When set to
true
, validates new objects added to association when saving the parent object.false
by default. If you want to ensure associated objects are revalidated on every update, usevalidates_associated
. - :autosave
-
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.
Note that
NestedAttributes::ClassMethods#accepts_nested_attributes_for
sets:autosave
totrue
. - :inverse_of
-
Specifies the name of the
belongs_to
association on the associated object that is the inverse of thishas_one
association. See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail. - :required
-
When set to
true
, the association will also have its presence validated. This will validate the association itself, not the id. You can use:inverse_of
to avoid an extra query during validation. - :strict_loading
-
Enforces strict loading every time the associated record is loaded through this association.
- :ensuring_owner_was
-
Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.
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, -> { order('posted_on') }, class_name: "Comment"
has_one :project_manager, -> { where(role: 'project_manager') }, class_name: "Person"
has_one :attachment, as: :attachable
has_one :boss, -> { readonly }
has_one :club, through: :membership
has_one :primary_address, -> { where(primary: true) }, through: :addressables, source: :addressable
has_one :credit_card, required: true
has_one :credit_card, strict_loading: true