simple_habtm.rb

class User < ActiveRecord::Base
  has_many :memberships, :dependent => :destroy
  has_many :groups, :through => :memberships
end

class Membership < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
end

class Group < ActiveRecord::Base
  has_many :memberships, :dependent => :destroy
  has_many :users, :through => :memberships
end