多対多なモデル
class Hoge< ActiveRecord::Base has_many :hoges_fugas_assigns has_many :fugas, :through => :hoges_fugas_assigns, :source => :wawawa_fuga #以下略
class WawawaFuga< ActiveRecord::Base has_many :hoges_fugas_assigns has_many :hoges, :through => :hoges_fugas_assigns #以下略
class HogesFugasAssign< ActiveRecord::Base belongs_to :hoge belongs_to :wawawa_fuga #以下略