[RoR] has_and_belongs_to_many

2.6 The has_and_belongs_to_many Association

A has_and_belongs_to_many association creates a direct many-to-many connection with another model, with no intervening model. For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way:

class Assembly < ActiveRecord::Base   has_and_belongs_to_many :parts end   class Part < ActiveRecord::Base   has_and_belongs_to_many :assemblies end

The corresponding migration might look like this:

class CreateAssembliesAndParts < ActiveRecord::Migration   def change     create_table :assemblies do |t|       t.string :name       t.timestamps null: false     end       create_table :parts do |t|       t.string :part_number       t.timestamps null: false     end       create_table :assemblies_parts, id: false do |t|       t.belongs_to :assembly, index: true       t.belongs_to :part, index: true     end   end end