Module: Sequel::Plugins::ManyThroughMany

Defined in:

lib/sequel/plugins/many_through_many.rb

Overview

The many_through_many plugin allow you to create an association to multiple objects using multiple join tables. For example, assume the following associations:

Artist.many_to_many :albums
Album.many_to_many :tags

The many_through_many plugin would allow this:

Artist.plugin :many_through_many
Artist.many_through_many :tags, [[:albums_artists, :artist_id, :album_id], [:albums, :id, :id], [:albums_tags, :album_id, :tag_id]]

Which will give you the tags for all of the artist’s albums.

Here are some more examples:

# Same as Artist.many_to_many :albums
Artist.many_through_many :albums, [[:albums_artists, :artist_id, :album_id]]

# All artists that are associated to any album that this artist is associated to
Artist.many_through_many :artists, [[:albums_artists, :artist_id, :album_id], [:albums, :id, :id], [:albums_artists, :album_id, :artist_id]]

# All albums by artists that are associated to any album that this artist is associated to
Artist.many_through_many :artist_albums, [[:albums_artists, :artist_id, :album_id], [:albums, :id, :id], \
 [:albums_artists, :album_id, :artist_id], [:artists, :id, :id], [:albums_artists, :artist_id, :album_id]], \
 :class=>:Album

# All tracks on albums by this artist
Artist.many_through_many :tracks, [[:albums_artists, :artist_id, :album_id], [:albums, :id, :id]], \
 :right_primary_key=>:album_id

Often you don’t want the current object to appear in the array of associated objects. This is easiest to handle via an :after_load hook:

Artist.many_through_many :artists, [[:albums_artists, :artist_id, :album_id], [:albums, :id, :id], [:albums_artists, :album_id, :artist_id]],
  :after_load=>proc{|artist, associated_artists| associated_artists.delete(artist)}

You can also handle it by adding a dataset block that excludes the current record (so it won’t be retrieved at all), but that won’t work when eagerly loading, which is why the :after_load proc is recommended instead.

It’s also common to not want duplicate records, in which case the :distinct option can be used:

Artist.many_through_many :artists, [[:albums_artists, :artist_id, :album_id], [:albums, :id, :id], [:albums_artists, :album_id, :artist_id]],
 :distinct=>true

Defined Under Namespace

Modules: ClassMethods, DatasetMethods Classes: ManyThroughManyAssociationReflection