app/models/relation_member.rb

   1 class RelationMember < ActiveRecord::Base
   2   self.table_name = "current_relation_members"
   3   self.primary_keys = "relation_id", "sequence_id"
   4
   5   belongs_to :relation, :foreign_key => :relation_id
   6   belongs_to :member, :polymorphic => true
   7 end