has_and_belongs_to_many

Specifies a many-to-many relationship with another class. This associates two classes via an intermediate join table. Unless the join table is explicitly specified as an option, it is guessed using the lexical order of the class names. So a join between Developer and Project will give the default join table name of “developers_projects” because “D” outranks “P”. Note that this precedence is calculated using the < operator for String. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables “paper_boxes” and “papers” to generate a join table name of “papers_paper_boxes” because of the length of the name “paper_boxes”, but it in fact generates a join table name of “paper_boxes_papers”. Be aware of this caveat, and use the custom :join_table option if you need to.

The join table should not have a primary key or a model associated with it. You must manually generate the join table with a migration such as this:

class CreateDevelopersProjectsJoinTable < ActiveRecord::Migration
  def change
    create_table :developers_projects, :id => false do |t|
      t.integer :developer_id
      t.integer :project_id
    end
  end
end

It’s also a good idea to add indexes to each of those columns to speed up the joins process. However, in MySQL it is advised to add a compound index for both of the columns as MySQL only uses one index per table during the lookup.

Adds the following methods for retrieval and query:

collection(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). Note that this operation instantly fires update sql without waiting for the save or update call on the parent object.

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.exists?(…)

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#projects.exists?(...)

• Developer#projects.build (similar to Project.new("developer_id" => id))

• Developer#projects.create (similar to c = Project.new("developer_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 to Project will use “person_id” as the default :foreign_key.

:association_foreign_key

Specify the foreign key used for the association on the receiving side of the association. By default this is guessed to be the name of the associated class in lower-case and “_id” suffixed. So if a Person class makes a has_and_belongs_to_many association to Project, the association will use “project_id” as the default :association_foreign_key.

: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.

:having

Combined with :group this can be used to filter the records that a GROUP BY returns. Uses the HAVING 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.

:autosave

If true, always save the associated objects or destroy them if marked for destruction, when saving the parent object. If false, never save or destroy the associated objects. By default, only save associated objects that are new records.

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}"