Rails ActiveQuery 接口

Question

我在尝试使用连接编写 Rails 查询时遇到问题。

我正在构建的是根据给定试卷的各个部分对候选答案进行分组的哈希。 （注意，我有一个问题池，其中一些问题被添加到试卷的不同部分）

我已经使用地图实现了我的解决方案：

exam_candidate.exam.question_paper.sections.includes(:questions).each do |section|
  if section.questions.present?
    section_question_hash[section] = candidate_answers.where(question_id: section.questions.map(&:id))
  end
end

由于使用上述方法会创建大量在后台运行的数据库查询，因此使用起来并不健康，因此我需要使用联接。另外，我可以编写一个 SQL 查询与

select b.name, group_concat(c.id) from sections b
left join question_papers_questions a on a.section_id = b.id 
left join candidate_answers c on a.question_id = c.question_id 
where a.question_paper_id = 3 and c.exam_candidate_id = 4 
group by (b.name)

但是当我在 Rails 中尝试相同的方法时，我遇到了很多问题。

这是我的模型结构：

class ExamCandidate < ActiveRecord::Base
  belongs_to :exam
  belongs_to :candidate
  has_many :candidate_answers, dependent: :delete_all
  accepts_nested_attributes_for :candidate_answers
end

class Exam < ActiveRecord::Base
  has_many :exam_candidates, dependent: :destroy
  has_many :candidates, through: :exam_candidates
  belongs_to :question_paper
end

class QuestionPaper < ActiveRecord::Base
  has_many :exams, dependent: :nullify
  has_many :exam_candidates, through: :exams
  has_many :questions, through: :question_papers_questions
  has_many :question_papers_questions
  has_many :sections, dependent: :destroy
end

class QuestionPapersQuestion < ActiveRecord::Base
  belongs_to :question
  belongs_to :question_paper
  belongs_to :section
end

class Question < ActiveRecord::Base
  has_many :candidate_answers, through: :answers
  has_many :exams, through: :question_papers
  has_many :exam_candidates, through: :exams
  has_many :question_papers_questions
  has_many :question_papers, through: :question_papers_questions
end

class Section < ActiveRecord::Base
  belongs_to :question_paper
  has_many :questions, through: :question_papers_questions
  has_many :question_papers_questions
end

class CandidateAnswer < ActiveRecord::Base
  belongs_to :exam_candidate
  belongs_to :question
end

我已经给了足够的时间，但是几乎是 Rails 的新手是我的劣势，如果有人可以尝试或提出一些建议，那将非常有帮助。

Answer 1

sections = exam.question_paper.sections.select("sections.name, group_concat(candidate_answers.id) as candidate_answer_ids")
  .joins(:question_papers_questions).joins("inner join candidate_answers on question_papers_questions.question_id = candidate_answers.question_id")
  .where(candidate_answers: {exam_candidate_id: id}, question_papers_questions: { question_paper_id: exam.question_paper_id })
  .group("sections.name")