Module | ActiveRecord::Associations::ClassMethods
In: | lib/active_record/associations.rb

Associations are a set of macro-like class methods for tying objects together through foreign keys. They express relationships like "Project has one Project Manager" or "Project belongs to a Portfolio". Each macro adds a number of methods to the class which are specialized according to the collection or association symbol and the options hash. It works much the same way as Ruby‘s own `attr*` methods. Example:

      class Project < ActiveRecord::Base
        belongs_to              :portfolio
        has_one                 :project_manager
        has_many                :milestones
        has_and_belongs_to_many :categories
      end

The project class now has the following methods (and more) to ease the traversal and manipulation of its relationships:

- `Project#portfolio, Project#portfolio=(portfolio), Project#portfolio.nil?`
- `Project#project_manager, Project#project_manager=(project_manager), Project#project_manager.nil?,`
- `Project#milestones.empty?, Project#milestones.size, Project#milestones, Project#milestones<<(milestone),` `Project#milestones.delete(milestone), Project#milestones.find(milestone_id), Project#milestones.find(:all, options),` `Project#milestones.build, Project#milestones.create`
- `Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1),` `Project#categories.delete(category1)`

### A word of warning

Don‘t create associations that have the same name as instance methods of ActiveRecord::Base. Since the association adds a method with that name to its model, it will override the inherited method and break things. For instance, `attributes` and `connection` would be bad choices for association names.

## Auto-generated methods

### Singular associations (one-to-one)

                                        |            |  belongs_to  |
      generated methods                 | belongs_to | :polymorphic | has_one
      ----------------------------------+------------+--------------+---------
      #other                            |     X      |      X       |    X
      #other=(other)                    |     X      |      X       |    X
      #build_other(attributes={})       |     X      |              |    X
      #create_other(attributes={})      |     X      |              |    X
      #other.create!(attributes={})     |            |              |    X
      #other.nil?                       |     X      |      X       |

### Collection associations (one-to-many / many-to-many)

                                        |       |          | has_many
      generated methods                 | habtm | has_many | :through
      ----------------------------------+-------+----------+----------
      #others                           |   X   |    X     |    X
      #others=(other,other,...)         |   X   |    X     |    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.exist?                    |   X   |    X     |    X
      #others.uniq                      |   X   |    X     |    X
      #others.reset                     |   X   |    X     |    X

## 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. Example:

      class User < ActiveRecord::Base
        # I reference an account.
        belongs_to :account
      end

      class Account < ActiveRecord::Base
        # One user references me.
        has_one :user
      end

The tables for these classes could look something like:

      CREATE TABLE users (
        id int(11) NOT NULL auto_increment,
        account_id int(11) default NULL,
        name varchar default NULL,
        PRIMARY KEY  (id)
      )

      CREATE TABLE accounts (
        id int(11) NOT NULL auto_increment,
        name varchar default NULL,
        PRIMARY KEY  (id)
      )

## Unsaved objects and associations

You can manipulate objects and associations before they are saved to the database, but there is some special behavior you should be aware of, mostly involving the saving of associated objects.

### One-to-one associations

- Assigning an object to a `has_one` association automatically saves that object and the object being replaced (if there is one), in order to update their primary keys - except if the parent object is unsaved (`new_record? == true`).
- If either of these saves fail (due to one of the objects being invalid) the assignment statement returns `false` and the assignment is cancelled.
- If you wish to assign an object to a `has_one` association without saving it, use the `association.build` method (documented below).
- Assigning an object to a `belongs_to` association does not save the object, since the foreign key field belongs on the parent. It does not save the parent either.

### Collections

- Adding an object to a collection (`has_many` or `has_and_belongs_to_many`) automatically saves that object, except if the parent object (the owner of the collection) is not yet stored in the database.
- If saving any of the objects being added to a collection (via `push` or similar) fails, then `push` returns `false`.
- You can add an object to a collection without automatically saving it by using the `collection.build` method (documented below).
- All unsaved (`new_record? == true`) members of the collection are automatically saved when the parent is saved.

### Association callbacks

Similar to the normal callbacks that hook into the lifecycle of an Active Record object, you can also define callbacks that get triggered when you add an object to or remove an object from an association collection. Example:

      class Project
        has_and_belongs_to_many :developers, :after_add => :evaluate_velocity

        def evaluate_velocity(developer)
          ...
        end
      end

It‘s possible to stack callbacks by passing them as an array. Example:

      class Project
        has_and_belongs_to_many :developers, :after_add => [:evaluate_velocity, Proc.new { |p, d| p.shipping_date = Time.now}]
      end

Possible callbacks are: `before_add`, `after_add`, `before_remove` and `after_remove`.

Should any of the `before_add` callbacks throw an exception, the object does not get added to the collection. Same with the `before_remove` callbacks; if an exception is thrown the object doesn‘t get removed.

### Association extensions

The proxy objects that control the access to associations can be extended through anonymous modules. This is especially beneficial for adding new finders, creators, and other factory-type methods that are only used as part of this association. Example:

      class Account < ActiveRecord::Base
        has_many :people do
          def find_or_create_by_name(name)
            first_name, last_name = name.split(" ", 2)
            find_or_create_by_first_name_and_last_name(first_name, last_name)
          end
        end
      end

      person = Account.find(:first).people.find_or_create_by_name("David Heinemeier Hansson")
      person.first_name # => "David"
      person.last_name  # => "Heinemeier Hansson"

If you need to share the same extensions between many associations, you can use a named extension module. Example:

      module FindOrCreateByNameExtension
        def find_or_create_by_name(name)
          first_name, last_name = name.split(" ", 2)
          find_or_create_by_first_name_and_last_name(first_name, last_name)
        end
      end

      class Account < ActiveRecord::Base
        has_many :people, :extend => FindOrCreateByNameExtension
      end

      class Company < ActiveRecord::Base
        has_many :people, :extend => FindOrCreateByNameExtension
      end

If you need to use multiple named extension modules, you can specify an array of modules with the `:extend` option. In the case of name conflicts between methods in the modules, methods in modules later in the array supercede those earlier in the array. Example:

      class Account < ActiveRecord::Base
        has_many :people, :extend => [FindOrCreateByNameExtension, FindRecentExtension]
      end

Some extensions can only be made to work with knowledge of the association proxy‘s internals. Extensions can access relevant state using accessors on the association proxy:

- `proxy_owner` - Returns the object the association is part of.
- `proxy_reflection` - Returns the reflection object that describes the association.
- `proxy_target` - Returns the associated object for `belongs_to` and `has_one`, or the collection of associated objects for `has_many` and `has_and_belongs_to_many`.

### Association Join Models

Has Many associations can be configured with the `:through` option to use an explicit join model to retrieve the data. This operates similarly to a `has_and_belongs_to_many` association. The advantage is that you‘re able to add validations, callbacks, and extra attributes on the join model. Consider the following schema:

      class Author < ActiveRecord::Base
        has_many :authorships
        has_many :books, :through => :authorships
      end

      class Authorship < ActiveRecord::Base
        belongs_to :author
        belongs_to :book
      end

      @author = Author.find :first
      @author.authorships.collect { |a| a.book } # selects all books that the author's authorships belong to.
      @author.books                              # selects all books by using the Authorship join model

You can also go through a `has_many` association on the join model:

      class Firm < ActiveRecord::Base
        has_many   :clients
        has_many   :invoices, :through => :clients
      end

      class Client < ActiveRecord::Base
        belongs_to :firm
        has_many   :invoices
      end

      class Invoice < ActiveRecord::Base
        belongs_to :client
      end

      @firm = Firm.find :first
      @firm.clients.collect { |c| c.invoices }.flatten # select all invoices for all clients of the firm
      @firm.invoices                                   # selects all invoices by going through the Client join model.

### Polymorphic Associations

Polymorphic associations on models are not restricted on what types of models they can be associated with. Rather, they specify an interface that a `has_many` association must adhere to.

      class Asset < ActiveRecord::Base
        belongs_to :attachable, :polymorphic => true
      end

      class Post < ActiveRecord::Base
        has_many :assets, :as => :attachable         # The :as option specifies the polymorphic interface to use.
      end

      @asset.attachable = @post

This works by using a type column in addition to a foreign key to specify the associated record. In the Asset example, you‘d need an `attachable_id` integer column and an `attachable_type` string column.

Using polymorphic associations in combination with single table inheritance (STI) is a little tricky. In order for the associations to work as expected, ensure that you store the base model for the STI models in the type column of the polymorphic association. To continue with the asset example above, suppose there are guest posts and member posts that use the posts table for STI. In this case, there must be a `type` column in the posts table.

      class Asset < ActiveRecord::Base
        belongs_to :attachable, :polymorphic => true

        def attachable_type=(sType)
           super(sType.to_s.classify.constantize.base_class.to_s)
        end
      end

      class Post < ActiveRecord::Base
        # because we store "Post" in attachable_type now :dependent => :destroy will work
        has_many :assets, :as => :attachable, :dependent => :destroy
      end

      class GuestPost < Post
      end

      class MemberPost < Post
      end

## Caching

All of the methods are built on a simple caching principle that will keep the result of the last query around unless specifically instructed not to. The cache is even shared across methods to make it even cheaper to use the macro-added methods without worrying too much about performance at the first go. Example:

      project.milestones             # fetches milestones from the database
      project.milestones.size        # uses the milestone cache
      project.milestones.empty?      # uses the milestone cache
      project.milestones(true).size  # fetches milestones from the database
      project.milestones             # uses the milestone cache

## Eager loading of associations

Eager loading is a way to find objects of a certain class and a number of named associations. 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. Example:

      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.find(:all, :include => [ :author, :comments ], :conditions => ['comments.approved = ?', true])

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 eagerload 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)

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)

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.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.

## 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. Example:

      module MyApplication
        module Business
          class Firm < ActiveRecord::Base; end
        end

        module Billing
          class Account < ActiveRecord::Base
            belongs_to :firm, :class_name => "MyApplication::Business::Firm"
          end
        end
      end

## Type safety with `ActiveRecord::AssociationTypeMismatch`

If you attempt to assign an object to an association that doesn‘t match the inferred or specified `:class_name`, you‘ll get an `ActiveRecord::AssociationTypeMismatch`.

## Options

All of the association macros can be specialized through options. This makes cases more complex than the simple and guessable ones possible.

### Methods

belongs_to has_and_belongs_to_many has_many has_one

### Public Instance methods

belongs_to(association_id, options = {})

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(force_reload = false)
  Returns the associated object. `nil` is returned if none is found.
association=(associate)
  Assigns the associate object, extracts the primary key, and sets it as the foreign key.
association.nil?
  Returns `true` if there is no associated object.
build_association(attributes = {})
  Returns a new object of the associated type that has been instantiated with `attributes` and linked to this object through a foreign key, but has not yet been saved.
create_association(attributes = {})
  Returns a new object of the associated type that has been instantiated with `attributes`, 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?`.)

### Example

A Post class declares `belongs_to :author`, which will add:

- `Post#author` (similar to `Author.find(author_id)`)
- `Post#author=(author)` (similar to `post.author_id = author.id`)
- `Post#author?` (similar to `post.author == some_author`)
- `Post#author.nil?`
- `Post#build_author` (similar to `post.author = Author.new`)
- `Post#create_author` (similar to `post.author = Author.new; post.author.save; post.author`)

The declaration can also include an options hash to specialize the behavior of the association.

### Options

:class_name
  Specify the class name of the association. Use it only if that name can‘t be inferred from the association name. So `has_one :author` will by default be linked to the Author class, but if the real class name is Person, you‘ll have to specify it with this option.
:conditions
  Specify the conditions that the associated object must meet in order to be included as a `WHERE` SQL fragment, such as `authorized = 1`.
:select
  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.
: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".
: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. 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.
:counter_cache
  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`.
:include
  Specify second-order associations that should be eager loaded when this object is loaded.
:polymorphic
  Specify this association is a polymorphic association by passing `true`. 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`).
:readonly
  If true, the associated object is readonly through the association.
:validate
  If false, don‘t validate the associated objects when saving the parent object. `false` by default.

Option examples:

      belongs_to :firm, :foreign_key => "client_of"
      belongs_to :author, :class_name => "Person", :foreign_key => "author_id"
      belongs_to :valid_coupon, :class_name => "Coupon", :foreign_key => "coupon_id",
                 :conditions => 'discounts > #{payments_count}'
      belongs_to :attachable, :polymorphic => true
      belongs_to :project, :readonly => true
      belongs_to :post, :counter_cache => true

has_and_belongs_to_many(association_id, options = {}, &extension)

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.

Deprecated: Any additional fields added to the join table will be placed as attributes when pulling records out through `has_and_belongs_to_many` associations. Records returned from join tables with additional attributes will be marked as readonly (because we can‘t save changes to the additional attributes). It‘s strongly recommended that you upgrade any associations with attributes to a real join model (see introduction).

Adds the following methods for retrieval and query:

collection(force_reload = false)
  Returns an array of all the associated objects. An empty array is returned if none are found.
collection<<(object, …)
  Adds one or more objects to the collection by creating associations in the join table (`collection.push` and `collection.concat` are aliases to this method).
collection.delete(object, …)
  Removes one or more objects from the collection by removing their associations from the join table. This does not destroy the objects.
collection=objects
  Replaces the collection‘s content by deleting and adding objects as appropriate.
collection_singular_ids
  Returns an array of the associated objects’ ids.
collection_singular_ids=ids
  Replace the collection by the objects identified by the primary keys 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 as ActiveRecord::Base.find.
collection.exist?(…)
  Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::Base.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` is replaced with the symbol passed as the first argument, so `has_and_belongs_to_many :categories` would add among others `categories.empty?`.)

### Example

A Developer class declares `has_and_belongs_to_many :projects`, which will add:

- `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#clients.exist?(…)`
- `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.

### 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 any `has_and_belongs_to_many` declaration in order to work.
:foreign_key
  Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a `has_and_belongs_to_many` association will use "person_id" as the default `:foreign_key`.
:association_foreign_key
  Specify the association foreign key used for the association. By default this is guessed to be the name of the associated class in lower-case and "_id" suffixed. So if the associated class is Project, the `has_and_belongs_to_many` association will use "project_id" as the default `:association_foreign_key`.
:conditions
  Specify the conditions that the associated object must meet in order to be included as a `WHERE` SQL fragment, such as `authorized = 1`. 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`.
:order
  Specify the order in which the associated objects are returned as an `ORDER BY` SQL fragment, such as `last_name, first_name DESC`
:uniq
  If true, duplicate associated objects will be ignored by accessors and query methods.
:finder_sql
  Overwrite the default generated SQL statement used to fetch the association with a manual statement
:counter_sql
  Specify a complete SQL statement to fetch the size of the association. If `:finder_sql` is specified but not `:counter_sql`, `:counter_sql` will be generated by replacing `SELECT … FROM` with `SELECT COUNT(*) FROM`.
:delete_sql
  Overwrite the default generated SQL statement used to remove links between the associated classes with a manual statement.
:insert_sql
  Overwrite the default generated SQL statement used to add links between the associated classes with a manual statement.
:extend
  Anonymous module for extending the proxy, see "Association extensions".
:include
  Specify second-order associations that should be eager loaded when the collection is loaded.
:group
  An attribute name by which the result should be grouped. Uses the `GROUP BY` SQL-clause.
:limit
  An integer determining the limit on the number of rows that should be returned.
:offset
  An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
:select
  By default, this is `*` as in `SELECT * FROM`, but can be changed if, 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.
:readonly
  If true, all the associated objects are readonly through the association.
:validate
  If false, don‘t validate the associated objects when saving the parent object. `true` by default.

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}'

has_many(association_id, options = {}, &extension)

Specifies a one-to-many association. The following methods for retrieval and query of collections of associated objects will be added:

collection(force_reload = false)
  Returns an array of all the associated objects. An empty array is returned if none are found.
collection<<(object, …)
  Adds one or more objects to the collection by setting their foreign keys to the collection‘s primary key.
collection.delete(object, …)
  Removes one or more objects from the collection by setting their foreign keys to `NULL`. Objects will be in addition destroyed if they‘re associated with `:dependent => :destroy`, and deleted if they‘re associated with `:dependent => :delete_all`.
collection=objects
  Replaces the collections content by deleting and adding objects as appropriate.
collection_singular_ids
  Returns an array of the associated objects’ ids
collection_singular_ids=ids
  Replace the collection with the objects identified by the primary keys in `ids`
collection.clear
  Removes every object from the collection. This destroys the associated objects if they are associated with `:dependent => :destroy`, deletes them directly from the database if `:dependent => :delete_all`, otherwise sets their foreign keys to `NULL`.
collection.empty?
  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::Base.find.
collection.exist?(…)
  Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::Base.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. Note: This only works if an associated object already exists, not if it‘s `nil`!
collection.create(attributes = {})
  Returns a new object of the collection type that has been instantiated with `attributes`, linked to this object through a foreign key, and that has already been saved (if it passed the validation). Note: This only works if an associated object already exists, not if it‘s `nil`!

(Note: `collection` is replaced with the symbol passed as the first argument, so `has_many :clients` would add among others `clients.empty?`.)

### Example

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.exist?(:name => ‘ACME’)` (similar to `Client.exist?(: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.

### Supported 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 the Product class, but if the real class name is SpecialProduct, you‘ll have to specify it with this option.
:conditions
  Specify the conditions that the associated objects must meet in order to be included as a `WHERE` SQL fragment, such as `price > 5 AND name LIKE ‘B%’`. 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`.
:order
  Specify the order in which the associated objects are returned as an `ORDER BY` SQL fragment, such as `last_name, first_name DESC`.
:foreign_key
  Specify the foreign key used for the association. By default this is guessed to be the name of 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`.
:primary_key
  Specify the method that returns the primary key used for the association. By default this is `id`.
:dependent
  If set to `:destroy` all the associated objects are destroyed alongside this object by calling their `destroy` method. If set to `:delete_all` all associated objects are deleted without calling their `destroy` method. If set to `:nullify` all associated objects’ foreign keys are set to `NULL` without calling their `save` callbacks. *Warning:* This option is ignored when also using the `:through` option.
:finder_sql
  Specify a complete SQL statement to fetch the association. This is a good way to go for complex associations that depend on multiple tables. Note: When this option is used, `find_in_collection` is not added.
:counter_sql
  Specify a complete SQL statement to fetch the size of the association. If `:finder_sql` is specified but not `:counter_sql`, `:counter_sql` will be generated by replacing `SELECT … FROM` with `SELECT COUNT(*) FROM`.
:extend
  Specify a named module for extending the proxy. See "Association extensions".
:include
  Specify second-order associations that should be eager loaded when the collection is loaded.
:group
  An attribute name by which the result should be grouped. Uses the `GROUP BY` SQL-clause.
:limit
  An integer determining the limit on the number of rows that should be returned.
:offset
  An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
:select
  By default, this is `*` as in `SELECT * FROM`, but can be changed if you, for example, want to do a join but not include the joined columns. Do not forget to include the primary and foreign keys, otherwise it will raise an error.
:as
  Specifies a polymorphic interface (See `belongs_to`).
:through
  Specifies a Join Model through which to perform the query. Options for `:class_name` and `:foreign_key` are ignored, as the association uses the source reflection. You can only use a `:through` query through a `belongs_to` or `has_many` association on the join model.
: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 polymorphic `belongs_to`.
:uniq
  If true, duplicates will be omitted from the collection. Useful in conjunction with `:through`.
:readonly
  If true, all the associated objects are readonly through the association.
:validate
  If false, don‘t validate the associated objects when saving the parent object. true by default.

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'

has_one(association_id, options = {})

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(force_reload = false)
  Returns the associated object. `nil` is returned if none is found.
association=(associate)
  Assigns the associate object, extracts the primary key, sets it as the foreign key, and saves the associate object.
association.nil?
  Returns `true` if there is no associated object.
build_association(attributes = {})
  Returns a new object of the associated type that has been instantiated with `attributes` and linked to this object through a foreign key, but has not yet been saved. Note: This ONLY works if an association already exists. It will NOT work if the association is `nil`.
create_association(attributes = {})
  Returns a new object of the associated type that has been instantiated with `attributes`, 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?`.)

### Example

An Account class declares `has_one :beneficiary`, which will add:

- `Account#beneficiary` (similar to `Beneficiary.find(:first, :conditions => "account_id = #{id}")`)
- `Account#beneficiary=(beneficiary)` (similar to `beneficiary.account_id = account.id; beneficiary.save`)
- `Account#beneficiary.nil?`
- `Account#build_beneficiary` (similar to `Beneficiary.new("account_id" => id)`)
- `Account#create_beneficiary` (similar to `b = Beneficiary.new("account_id" => id); b.save; b`)

### 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.
:conditions
  Specify the conditions that the associated object must meet in order to be included as a `WHERE` SQL fragment, such as `rank = 5`.
:order
  Specify the order in which the associated objects are returned as an `ORDER BY` SQL fragment, such as `last_name, first_name DESC`.
: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 `:nullify`, the associated object‘s foreign key is set to `NULL`. Also, association is assigned.
:foreign_key
  Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a Person class that makes a `has_one` association will use "person_id" as the default `:foreign_key`.
:primary_key
  Specify the method that returns the primary key used for the association. By default this is `id`.
:include
  Specify second-order associations that should be eager loaded when this object is loaded.
:as
  Specifies a polymorphic interface (See `belongs_to`).
:select
  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.
:through
  Specifies a Join Model through which to perform the query. Options for `:class_name` and `:foreign_key` are ignored, as the association uses the source reflection. You can only use a `:through` query through a `has_one` or `belongs_to` association on the join model.
: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 polymorphic `belongs_to`.
:readonly
  If true, the associated object is readonly through the association.
:validate
  If false, don‘t validate the associated object when saving the parent object. `false` by default.

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

[Validate]