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

## Example

## Is it belongs_to or has_one?

Both express a 1-1 relationship, the difference is mostly where to place the foreign key, which goes on the table for the class saying belongs_to. Example:

      class 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 behaviour you should be aware of, mostly involving the saving of associated objects.

### One-to-one associations

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

### Collections

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

### Association callbacks

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

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

        def evaluate_velocity(developer)
          ...
        end
      end

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

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

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

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

### Association extensions

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

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

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

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

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

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

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

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 <tt>:as</tt> option specifies the polymorphic interface to use.
      end

      @asset.attachable = @post

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

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. So there will be an additional ‘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 < ActiveRecord::Base
      end

      class MemberPost < ActiveRecord::Base
      end

## Caching

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

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

## Eager loading of associations

Eager loading is a way to find objects of a certain class and a number of named associations along with it in a single SQL call. This is one of the easiest ways of to prevent the dreaded 1+N problem in which fetching 100 posts that each needs to display their author triggers 101 database queries. Through the use of eager loading, the 101 queries can be reduced to 1. Example:

      class Post < ActiveRecord::Base
        belongs_to :author
        has_many   :comments
      end

Consider the following loop using the class above:

      for post in Post.find(:all)
        puts "Post:            " + post.title
        puts "Written by:      " + post.author.name
        puts "Last comment on: " + post.comments.first.created_on
      end

To iterate over these one hundred posts, we‘ll generate 201 database queries. Let‘s first just optimize it for retrieving the author:

      for post in Post.find(:all, :include => :author)

This references the name of the belongs_to association that also used the :author symbol, so the find will now weave in a join something like this: LEFT OUTER JOIN authors ON authors.id = posts.author_id. Doing so will cut down the number of queries from 201 to 101.

We can improve upon the situation further by referencing both associations in the finder with:

      for post in Post.find(:all, :include => [ :author, :comments ])

That‘ll add another join along the lines of: LEFT OUTER JOIN comments ON comments.post_id = posts.id. And we‘ll be down to 1 query. But that shouldn‘t fool you to think that you can pull out huge amounts of data with no performance penalty just because you‘ve reduced the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So it‘s no catch-all for performance problems, but it‘s a great way to cut down on the number of queries in a situation as the one described above.

Since the eager loading pulls from multiple tables, you‘ll have to disambiguate any column references in both conditions and orders. So :order => "posts.id DESC" will work while :order => "id DESC" will not. Because eager loading generates the SELECT statement too, the :select option is ignored.

You can use eager loading on multiple associations from the same table, but you cannot use those associations in orders and conditions as there is currently not any way to disambiguate them. Eager loading will not pull additional attributes on join tables, so "rich associations" with has_and_belongs_to_many are not a good fit for eager loading.

When eager loaded, conditions are interpolated in the context of the model class, not the model instance. Conditions are lazily interpolated before the actual model exists.

## Table Aliasing

ActiveRecord uses table aliasing in the case that a table is referenced multiple times in a join. If a table is referenced only once, the standard table name is used. The second time, the table is aliased as #{reflection_name}_#{parent_table_name}. Indexes are appended for any more successive uses of the table name.

      Post.find :all, :include => :comments
      # => SELECT ... FROM posts LEFT OUTER JOIN comments ON ...
      Post.find :all, :include => :special_comments # STI
      # => SELECT ... FROM posts LEFT OUTER JOIN comments ON ... AND comments.type = 'SpecialComment'
      Post.find :all, :include => [:comments, :special_comments] # special_comments is the reflection name, posts is the parent table name
      # => SELECT ... FROM posts LEFT OUTER JOIN comments ON ... LEFT OUTER JOIN comments special_comments_posts

Acts as tree example:

      TreeMixin.find :all, :include => :children
      # => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
      TreeMixin.find :all, :include => {:children => :parent} # using cascading eager includes
      # => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
                                  LEFT OUTER JOIN parents_mixins ...
      TreeMixin.find :all, :include => {:children => {:parent => :children}}
      # => SELECT ... FROM mixins LEFT OUTER JOIN mixins childrens_mixins ...
                                  LEFT OUTER JOIN parents_mixins ...

LEFT OUTER JOIN mixins childrens_mixins_2

Has and Belongs to Many join tables use the same idea, but add a _join suffix:

      Post.find :all, :include => :categories
      # => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
      Post.find :all, :include => {:categories => :posts}
      # => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
                                 LEFT OUTER JOIN categories_posts posts_categories_join LEFT OUTER JOIN posts posts_categories
      Post.find :all, :include => {:categories => {:posts => :categories}}
      # => SELECT ... FROM posts LEFT OUTER JOIN categories_posts ... LEFT OUTER JOIN categories ...
                                 LEFT OUTER JOIN categories_posts posts_categories_join LEFT OUTER JOIN posts posts_categories
                                 LEFT OUTER JOIN categories_posts categories_posts_join LEFT OUTER JOIN categories categories_posts

If you wish to specify your own custom joins using a :joins option, those table names will take precedence over the eager associations..

      Post.find :all, :include => :comments, :joins => "inner join comments ..."
      # => SELECT ... FROM posts LEFT OUTER JOIN comments_posts ON ... INNER JOIN comments ...
      Post.find :all, :include => [:comments, :special_comments], :joins => "inner join comments ..."
      # => SELECT ... FROM posts LEFT OUTER JOIN comments comments_posts ON ...
                                 LEFT OUTER JOIN comments special_comments_posts ...
                                 INNER JOIN comments ...

Table aliases are automatically truncated according to the maximum length of table identifiers according to the specific database.

## Modules

By default, associations will look for objects within the current module scope. Consider:

      module MyApplication
        module Business
          class Firm < ActiveRecord::Base
             has_many :clients
           end

          class Company < ActiveRecord::Base; end
        end
      end

When Firm#clients is called, it‘ll in turn call `MyApplication::Business::Company.find(firm.id)`. If you want to associate with a class in another module scope this can be done by specifying the complete class name, such as:

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

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

## Type safety with ActiveRecord::AssociationTypeMismatch

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

## Options

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

### Methods

belongs_to has_and_belongs_to_many has_many has_one

### Public Instance methods

belongs_to(association_id, options = {})

Adds the following methods for retrieval and query for a single associated object that this object holds an id to. `association` is replaced with the symbol passed as the first argument, so `belongs_to :author` would add among others `author.nil?`.

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

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

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

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

Options are:

- `:class_name` - specify the class name of the association. Use it only if that name can‘t be inferred from the association name. So `has_one :author` will by default be linked to the `Author` class, but if the real class name is `Person`, you‘ll have to specify it with this option.
- `:conditions` - specify the conditions that the associated object must meet in order to be included as a "WHERE" sql fragment, such as "authorized = 1".
- `:order` - specify the order from which the associated object will be picked at the top. Specified as an "ORDER BY" sql fragment, such as "last_name, first_name DESC"
- `:foreign_key` - specify the foreign key used for the association. By default this is guessed to be the name of the associated class in lower-case and "_id" suffixed. So a `Person` class that makes a belongs_to association to a `Boss` class will use "boss_id" as the default foreign_key.
- `:counter_cache` - caches the number of belonging objects on the associate class through use of increment_counter and decrement_counter. The counter cache is incremented when an object of this class is created and decremented when it‘s destroyed. This requires that a column named "#{table_name}_count" (such as comments_count for a belonging Comment class) is used on the associate class (such as a Post class). You can also specify a custom counter cache column by given that name instead of a true/false value to this option (e.g., `:counter_cache => :my_custom_counter`.)
- `:include` - specify second-order associations that should be eager loaded when this object is loaded.
- `:polymorphic` - specify this association is a polymorphic association by passing true.

Option examples:

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

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

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 attrbutes). It‘s strongly recommended that you upgrade any associations with attributes to a real join model (see introduction).

Adds the following methods for retrieval and query. `collection` is replaced with the symbol passed as the first argument, so `has_and_belongs_to_many :categories` would add among others `categories.empty?`.

- `collection(force_reload = false)` - returns an array of all the associated objects. An empty array is returned if none is found.
- `collection<<(object, …)` - adds one or more objects to the collection by creating associations in the join table (collection.push and collection.concat are aliases to this method).
- `collection.push_with_attributes(object, join_attributes)` - adds one to the collection by creating an association in the join table that also holds the attributes from `join_attributes` (should be a hash with the column names as keys). This can be used to have additional attributes on the join, which will be injected into the associated objects when they are retrieved through the collection. (collection.concat_with_attributes is an alias to this method). This method is now deprecated.
- `collection.delete(object, …)` - removes one or more objects from the collection by removing their associations from the join table. This does not destroy the objects.
- `collection=objects` - replaces the collections content by deleting and adding objects as appropriate.
- `collection_singular_ids` - 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.
- `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` and linked to this object through the join table and that has already been saved (if it passed the validation).

Example: An Developer class declares `has_and_belongs_to_many :projects`, which will add:

- `Developer#projects`
- `Developer#projects<<`
- `Developer#projects.delete`
- `Developer#projects=`
- `Developer#project_ids`
- `Developer#project_ids=`
- `Developer#projects.clear`
- `Developer#projects.empty?`
- `Developer#projects.size`
- `Developer#projects.find(id)`
- `Developer#projects.build` (similar 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 are:

- `:class_name` - specify the class name of the association. Use it only if that name can‘t be inferred from the association name. So `has_and_belongs_to_many :projects` will by default be linked to the `Project` class, but if the real class name is `SuperProject`, you‘ll have to specify it with this option.
- `:join_table` - specify the name of the join table if the default based on lexical order isn‘t what you want. WARNING: If you‘re overwriting the table name of either class, the table_name method MUST be declared underneath any has_and_belongs_to_many declaration in order to work.
- `:foreign_key` - specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a `Person` class that makes a has_and_belongs_to_many association will use "person_id" as the default foreign_key.
- `:association_foreign_key` - specify the association foreign key used for the association. By default this is guessed to be the name of the associated class in lower-case and "_id" suffixed. So if the associated class is `Project`, the has_and_belongs_to_many association will use "project_id" as the default association foreign_key.
- `:conditions` - specify the conditions that the associated object must meet in order to be included as a "WHERE" sql fragment, such as "authorized = 1".
- `:order` - specify the order in which the associated objects are returned as a "ORDER BY" sql fragment, such as "last_name, first_name DESC"
- `:uniq` - if set to true, duplicate associated objects will be ignored by accessors and query methods
- `:finder_sql` - overwrite the default generated SQL used to fetch the association with a manual one
- `:delete_sql` - overwrite the default generated SQL used to remove links between the associated classes with a manual one
- `:insert_sql` - overwrite the default generated SQL used to add links between the associated classes with a manual one
- `:extend` - anonymous module for extending the proxy, see "Association extensions".
- `:include` - specify second-order associations that should be eager loaded when the collection is loaded.
- `:group`: An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.
- `:limit`: An integer determining the limit on the number of rows that should be returned.
- `:offset`: An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
- `:select`: By default, this is * as in SELECT * FROM, but can be changed if you for example want to do a join, but not include the joined columns.

Option examples:

      has_and_belongs_to_many :projects
      has_and_belongs_to_many :projects, :include => [ :milestones, :manager ]
      has_and_belongs_to_many :nations, :class_name => "Country"
      has_and_belongs_to_many :categories, :join_table => "prods_cats"
      has_and_belongs_to_many :active_projects, :join_table => 'developers_projects', :delete_sql =>
      'DELETE FROM developers_projects WHERE active=1 AND developer_id = #{id} AND project_id = #{record.id}'

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

Adds the following methods for retrieval and query of collections of associated objects. `collection` is replaced with the symbol passed as the first argument, so `has_many :clients` would add among others `clients.empty?`.

- `collection(force_reload = false)` - returns an array of all the associated objects. An empty array is returned if none are found.
- `collection<<(object, …)` - adds one or more objects to the collection by setting their foreign keys to the collection‘s primary key.
- `collection.delete(object, …)` - removes one or more objects from the collection by setting their foreign keys to NULL. This will also destroy the objects if they‘re declared as belongs_to and dependent on this model.
- `collection=objects` - replaces the collections content by deleting and adding objects as appropriate.
- `collection_singular_ids` - 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 destroys the associated objects if they are `:dependent`, deletes them directly from the database if they are `:dependent => :delete_all`, and sets their foreign keys to NULL otherwise.
- `collection.empty?` - returns true if there are no associated objects.
- `collection.size` - returns the number of associated objects.
- `collection.find` - finds an associated object according to the same rules as Base.find.
- `collection.build(attributes = {})` - returns a new object of the collection type that has been instantiated with `attributes` and linked to this object through a foreign key but has not yet been saved. *Note:* This only works if an associated object already exists, not if it‘s nil!
- `collection.create(attributes = {})` - returns a new object of the collection type that has been instantiated with `attributes` and linked to this object through a foreign key and that has already been saved (if it passed the validation). *Note:* This only works if an associated object already exists, not if it‘s nil!

Example: A Firm class declares `has_many :clients`, which will add:

- `Firm#clients` (similar to `Clients.find :all, :conditions => "firm_id = #{id}"`)
- `Firm#clients<<`
- `Firm#clients.delete`
- `Firm#clients=`
- `Firm#client_ids`
- `Firm#client_ids=`
- `Firm#clients.clear`
- `Firm#clients.empty?` (similar to `firm.clients.size == 0`)
- `Firm#clients.size` (similar to `Client.count "firm_id = #{id}"`)
- `Firm#clients.find` (similar to `Client.find(id, :conditions => "firm_id = #{id}")`)
- `Firm#clients.build` (similar to `Client.new("firm_id" => id)`)
- `Firm#clients.create` (similar to `c = Client.new("firm_id" => id); c.save; c`)

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

Options are:

- `:class_name` - specify the class name of the association. Use it only if that name can‘t be inferred from the association name. So `has_many :products` will by default be linked to the `Product` class, but if the real class name is `SpecialProduct`, you‘ll have to specify it with this option.
- `:conditions` - specify the conditions that the associated objects must meet in order to be included as a "WHERE" sql fragment, such as "price > 5 AND name LIKE ‘B%’".
- `:order` - specify the order in which the associated objects are returned as a "ORDER BY" sql fragment, such as "last_name, first_name DESC"
- `:group` - specify the attribute by which the associated objects are returned as a "GROUP BY" sql fragment, such as "category"
- `:foreign_key` - specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and "_id" suffixed. So a `Person` class that makes a has_many association will use "person_id" as the default foreign_key.
- `:dependent` - if set to :destroy all the associated objects are destroyed alongside this object by calling their destroy method. If set to :delete_all all associated objects are deleted without calling their destroy method. If set to :nullify all associated objects’ foreign keys are set to NULL without calling their save callbacks. NOTE: :dependent => true is deprecated and has been replaced with :dependent => :destroy. May not be set if :exclusively_dependent is also set.
- `:exclusively_dependent` - Deprecated; equivalent to :dependent => :delete_all. If set to true all the associated object are deleted in one SQL statement without having their before_destroy callback run. This should only be used on associations that depend solely on this class and don‘t need to do any clean-up in before_destroy. The upside is that it‘s much faster, especially if there‘s a counter_cache involved. May not be set if :dependent is also set.
- `:finder_sql` - specify a complete SQL statement to fetch the association. This is a good way to go for complex associations that depend on multiple tables. Note: When this option is used, `find_in_collection` is not added.
- `:counter_sql` - specify a complete SQL statement to fetch the size of the association. If +:finder_sql+ is specified but +:counter_sql+, +:counter_sql+ will be generated by replacing SELECT … FROM with SELECT COUNT(*) FROM.
- `:extend` - specify a named module for extending the proxy, see "Association extensions".
- `:include` - specify second-order associations that should be eager loaded when the collection is loaded.
- `:group`: An attribute name by which the result should be grouped. Uses the GROUP BY SQL-clause.
- `:limit`: An integer determining the limit on the number of rows that should be returned.
- `:offset`: An integer determining the offset from where the rows should be fetched. So at 5, it would skip the first 4 rows.
- `:select`: By default, this is * as in SELECT * FROM, but can be changed if you for example want to do a join, but not include the joined columns.
- `:as`: Specifies a polymorphic interface (See belongs_to).
- `:through`: Specifies a Join Model to perform the query through. Options for `:class_name` and `:foreign_key` are ignored, as the association uses the source reflection. You can only use a `:through` query through a `belongs_to` or `has_many` association.
- `:source`: Specifies the source association name used by `has_many :through` queries. Only use it if the name cannot be inferred from the association. `has_many :subscribers, :through => :subscriptions` will look for either +:subscribers+ or +:subscriber+ on `Subscription`, unless a +:source+ is given.
- `: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 set to true, duplicates will be omitted from the collection. Useful in conjunction with :through.

Option examples:

      has_many :comments, :order => "posted_on"
      has_many :comments, :include => :author
      has_many :people, :class_name => "Person", :conditions => "deleted = 0", :order => "name"
      has_many :tracks, :order => "position", :dependent => :destroy
      has_many :comments, :dependent => :nullify
      has_many :tags, :as => :taggable
      has_many :subscribers, :through => :subscriptions, :source => :user
      has_many :subscribers, :class_name => "Person", :finder_sql =>
          'SELECT DISTINCT people.* ' +
          'FROM people p, post_subscriptions ps ' +
          'WHERE ps.post_id = #{id} AND ps.person_id = p.id ' +
          'ORDER BY p.first_name'

has_one(association_id, options = {})

Adds the following methods for retrieval and query of a single associated object. `association` is replaced with the symbol passed as the first argument, so `has_one :manager` would add among others `manager.nil?`.

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

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

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

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

Options are:

- `:class_name` - specify the class name of the association. Use it only if that name can‘t be inferred from the association name. So `has_one :manager` will by default be linked to the `Manager` class, but if the real class name is `Person`, you‘ll have to specify it with this option.
- `:conditions` - specify the conditions that the associated object must meet in order to be included as a "WHERE" sql fragment, such as "rank = 5".
- `:order` - specify the order from which the associated object will be picked at the top. Specified as

       an "ORDER BY" sql fragment, such as "last_name, first_name DESC"

- `:dependent` - if set to :destroy (or true) 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.
- `:include` - specify second-order associations that should be eager loaded when this object is loaded.
- `:as`: Specifies a polymorphic interface (See belongs_to).

Option examples:

      has_one :credit_card, :dependent => :destroy  # destroys the associated credit card
      has_one :credit_card, :dependent => :nullify  # updates the associated records foriegn key value to null rather than destroying it
      has_one :last_comment, :class_name => "Comment", :order => "posted_on"
      has_one :project_manager, :class_name => "Person", :conditions => "role = 'project_manager'"
      has_one :attachment, :as => :attachable

[Validate]