Ruby on rails 3 Rails 3.2在销毁前有条件地销毁儿童不开火

我有以下三种型号：

class Tag < AR::Base
  has_many   :mention_tags, dependent: :destroy
  has_many   :mentions, through: :mention_tags

  before_destroy :destroy_mentions

  def destroy_mentions
    mentions.each do |mention|
      mention.destroy if mention.tags.count == 1
    end
  end
end

class MentionTag < AR::Base
  belongs_to :tag
  belongs_to :mention
end

class Mention < AR::Base
  has_many    :mention_tags
  has_many    :tags,         through: :mention_tags
end

而该规范不符合：

describe 'mention with only this tag' do
  before do
    @tag = Tag.create(text: 'whatever', conditional: 'yes')
    @mention = Mention.create(text: 'whatever', tags: [@tag])
  end

  it 'should delete the mention when deleting the tag' do
    @tag.destroy
    Mention.exists?(id: @mention.id).should be_false
  end
end

---

似乎注释掉

标签上的
dependent:：destroy
会导致所有规范都通过，但我想在删除标签时去掉所有这些规范，因为提及可能会有多个标签，并且不会总是被删除。
describe 'mention with only this tag' do
  before do
    @tag = Tag.create(text: 'whatever', conditional: 'yes')
    @mention = Mention.create(text: 'whatever', tags: [@tag])
  end

  it 'should delete the mention when deleting the tag' do
    @tag.destroy
    Mention.exists?(id: @mention.id).should be_false
  end
end