Rails 按两个属性计数和分组

Question

以下模型星座：

class Attempt < ApplicationRecord
  belongs_to :user
  belongs_to :answer
  belongs_to :question
end

class Answer < ApplicationRecord
  belongs_to :question
end

class Question < ApplicationRecord
  has_many :answers
  has_many :attempts
end

一个问题有多个答案，其中一个被标记为正确（correct 列）。用户尝试猜测问题的正确答案。

挑战：我想知道对于每个问题的用户，有多少次尝试成功（选择了正确答案）以及失败了多少次，而无需对数据库发起多个/多个查询。

让我最接近满意答案的是Question.joins(attempts: [:answer]).group('(attempts.question_id, answers.correct)').count（到目前为止忽略了那里的用户）。生成的 SQL：

SELECT COUNT(*) AS count_all, (attempts.question_id, answers.correct) AS attempts_question_id_answers_correct FROM "questions" INNER JOIN "attempts" ON "attempts"."question_id" = "questions"."id" INNER JOIN "answers" ON "answers"."id" = "attempts"."answer_id" GROUP BY (attempts.question_id, answers.correct)

这些值是正确的，但是这些“(1,f) 等”的结果看起来很奇怪。键：

{"(1,f)"=>4, "(3,t)"=>2, "(2,t)"=>4, "(2,f)"=>11, "(3,f)"=>6, "(1,t)"=>4}

我将不胜感激，尤其是在结果集方面的帮助 :)。提前非常感谢！

Answer 1

你试过了吗：

Question.joins(attempts: [:answer]).select("attempts.question_id as question_id, answers.correct as correct_answer").group('(attempts.question_id, answers.correct)').count