两个左外部联接查询不明确的列名

我正在为一个“问答”论坛制作rails 4应用程序。我有一个模型Micropost。我有一个模型QuestionAnswer,它将不同的微柱作为问题和答案连接起来:

Class Micropost < ActiveRecord::Base
  has_many question_answers, foreign_key: "question_id"
  has_many answers, through: :question_answers
  has_one :reverse_question_answer, foreign_key: "answer_id", class_name: "QuestionAnswer"
  has_one :question, through: :reverse_question_answer
end

Class QuestionAnswer < ActiveRecord::Base
  belongs_to :question, class_name: "Micropost"
  belongs_to :answer,   class_name: "Micropost"
end

现在我想做一个数据库查询。我想使用两个LEFT OUTER JOIN来创建一个包含所有问题和答案的联合表:

Micropost.
  joins('LEFT OUTER JOIN question_answers ON question_answers.question_id = microposts.id').
  joins('LEFT OUTER JOIN microposts ON microposts.id = question_answers.answer_id')

我收到错误消息

SQLite3::SQLException: ambiguous column name: microposts.id SELECT "microposts".* FROM "microposts" LEFT OUTER JOIN question_answers ON question_answers.question_id = microposts.id LEFT OUTER JOIN microposts ON microposts.id = question_answers.answer_id ORDER BY created_at DESC

我在上面的第二个joins命令中用答案替换了micropost,但它抱怨没有这样的表。我如何进行两次联接-谢谢