ruby-on-rails - 为模型编写迁移

Question

我正在尝试创建一个模型，在该模型中我将在两个位置使用相同的资源：

class AttachmentFileComment < ActiveRecord::Base

attr_accessible :action_type

belongs_to :attachments, :conditions=>['attachments.attachable_type=?', 'project']
belongs_to :users
belongs_to :approval_level
belongs_to :comments
belongs_to :attachments, :as=> :file_id, :conditions=>['attachments.attachable_type=?', 'attachment']

end

对于上面我如何编写迁移