has_and_belongs_to_many(name, scope = nil, **options, &extension) public

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

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

class CreateDevelopersProjectsJoinTable < ActiveRecord::Migration[6.0]
  def change
    create_join_table :developers, :projects

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

Adds the following methods for retrieval and query:

collection is a placeholder for the symbol passed as the name argument, so has_and_belongs_to_many :categories would add among others categories.empty?.


Returns a Relation of all the associated objects. An empty Relation 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, unless the parent object is a new record.

collection.delete(object, …)

Removes one or more objects from the collection by removing their associations from the join table. This does not destroy the objects.

collection.destroy(object, …)

Removes one or more objects from the collection by running destroy on each association in the join table, overriding any dependent option. This does not destroy the objects.


Replaces the collection’s content by deleting and adding objects as appropriate.


Returns an array of the associated objects’ ids.


Replace the collection by the objects identified by the primary keys in ids.


Removes every object from the collection. This does not destroy the objects.


Returns true if there are no associated objects.


Returns the number of associated objects.


Finds an associated object responding to the id and that meets the condition that it has to be associated with this object. Uses the same rules as ActiveRecord::FinderMethods#find.


Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::FinderMethods#exists?.

collection.build(attributes = {})

Returns a new object of the collection type that has been instantiated with attributes and linked to this object through the join table, but has not yet been saved.

collection.create(attributes = {})

Returns a new object of the collection type that has been instantiated with attributes, linked to this object through the join table, and that has already been saved (if it passed the validation).


Returns a Relation of all of the associated objects, forcing a database read. An empty Relation is returned if none are found.


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

  • Developer#projects

  • Developer#projects<<

  • Developer#projects.delete

  • Developer#projects.destroy

  • Developer#projects=

  • Developer#project_ids

  • Developer#project_ids=

  • Developer#projects.clear

  • Developer#projects.empty?

  • Developer#projects.size

  • Developer#projects.find(id)

  • Developer#projects.exists?(...)

  • Developer#projects.build (similar to Project.new(developer_id: id))

  • Developer#projects.create (similar to c = Project.new(developer_id: id); c.save; c)

  • Developer#projects.reload

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


You can pass a second argument scope as a callable (i.e. proc or lambda) to retrieve a specific set of records or customize the generated query when you access the associated collection.

Scope examples:

has_and_belongs_to_many :projects, -> { includes(:milestones, :manager) }
has_and_belongs_to_many :categories, ->(post) {
  where("default_category = ?", post.default_category)


The extension argument allows you to pass a block into a has_and_belongs_to_many association. This is useful for adding new finders, creators and other factory-type methods to be used as part of the association.

Extension examples:

has_and_belongs_to_many :contractors do
  def find_or_create_by_name(name)
    first_name, last_name = name.split(" ", 2)
    find_or_create_by(first_name: first_name, last_name: last_name)



Specify the class name of the association. Use it only if that name can’t be inferred from the association name. So has_and_belongs_to_many :projects will by default be linked to the Project class, but if the real class name is SuperProject, you’ll have to specify it with this option.


Specify the name of the join table if the default based on lexical order isn’t what you want. WARNING: If you’re overwriting the table name of either class, the table_name method MUST be declared underneath any #has_and_belongs_to_many declaration in order to work.


Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a #has_and_belongs_to_many association to Project will use “person_id” as the default :foreign_key.

If you are going to modify the association (rather than just read from it), then it is a good idea to set the :inverse_of option.


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.


When set to true, validates new objects added to association when saving the parent object. true by default. If you want to ensure associated objects are revalidated on every update, use validates_associated.


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

Note that NestedAttributes::ClassMethods#accepts_nested_attributes_for sets :autosave to true.


Enforces strict loading every time an associated record is loaded through this association.

Option examples:

has_and_belongs_to_many :projects
has_and_belongs_to_many :projects, -> { includes(:milestones, :manager) }
has_and_belongs_to_many :nations, class_name: "Country"
has_and_belongs_to_many :categories, join_table: "prods_cats"
has_and_belongs_to_many :categories, -> { readonly }
has_and_belongs_to_many :categories, strict_loading: true
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April 21, 2009
7 thanks

Do not forget to add indexes

Don’t forget to add indexes to HATM table:

add_index :developers_projects, [:developer_id, :project_id]
September 25, 2008
4 thanks

has_many :through

It’s is recommended to use has_many :through association instead of has_and_belongs_to_many. has_many :through is better supported and generally easier to work with once you grasp the idea.

June 25, 2008
4 thanks


The created association method also supports the ‘exists?’ method, similar to ActiveRecord::Base#exists?

has_and_belongs_to_many :categories
categories.exist?(1)   # Check whether there's a relation with a Category
                       # object whose id is 1.
categories.exist?(:id => 1)    # ditto
categories.exist?(['id', 1])   # ditto
categories.exist?(:name => 'Anime')
May 6, 2010
1 thank


Be aware that has_and_belongs_to_many saves association to join table immediately after assign. It does NOT wait for my_object.save. Hence if save does not get through validations (or fail for any other reason), associated records will still be in the database.

Here is a nice workaround: http://github.com/TylerRick/has_and_belongs_to_many_with_deferred_save

February 12, 2009
0 thanks

using collection=objects

It will fire one insert query per new record

March 24, 2009
0 thanks

Finding all records WITHOUT associations

(Thanks to someone on the rails IRC channel who gave me this tip.)

Where Users and Events have a habtm relationship, to find all Users that have no events:

User.find(:all, :include => :events, :conditions => { "events_users.event_id" => nil})

(Note that when specifying a condition on a joined table, you have to put the field name in a string rather than a symbol. In the above example, :events_users.event_id will not work.)