simple has_and_belongs_to_many example (from memory...)

create_join_table_for_domains_and_users.rb

class CreateJoinTableForDomainsAndUsers < ActiveRecord::Migration
  def change
    create_table :domains_users, id: false do |t|
      t.integer :user_id
      t.integer :domain_id
    end
    add_index :domains_users, [:user_id, :domain_id]
    add_index :domains_users, [:domain_id, :user_id]
  end
end

domain.rb

class Domain < ActiveRecord::Base
  has_and_belongs_to_many :users
end

user.rb

class User < ActiveRecord::Base
  has_and_belongs_to_many :domains
end