habtm.rb

##Class

class User
  has_and_belongs_to_many :groups
end

class Group
  has_and_belongs_to_many :groups
end

##Migration

class CreateUsersGroupsJoinTable < ActiveRecord::Migration
  def change
    create_table :users_groups, id: false do |t|
      t.integer :user_id, index: true
      t.integer :group_id, index: true
    end
  end
end


user.groups
group.users