Question

I have these model:

class Question
  has_many :answers
end

class Answer
  belongs_to :question
end

class Exam
  belongs_to :general_exam
  belongs_to :user
  has_many :questions, through: :exam_questions
end

class ExamQuestion
  belongs_to :exam
  belongs_to :question
end

Currently, I want to get all questions in Exam and answers of questions, so I used Specifying Conditions on Eager Loaded Associations, I ran this in console:

exam = Exam.find(16)
questions = Question.includes(:answers).where("id = ?", exam.question_ids)

Output in console after run questions = ...:

SELECT "questions".id FROM "questions" INNER JOIN "exam_questions" ON "questions"."id" = "exam_questions"."question_id" WHERE "exam_questions"."exam_id" = 16 ORDER BY questions.created_at DESC
  Question Load (0.8ms)  SELECT "questions".* FROM "questions" WHERE (id = 170,162,160) ORDER BY questions.created_at DESC
=> #<ActiveRecord::Relation:0x4c07ebc>

The first strange thing is, I saw in query, it did a INNER JOIN, but in rails guide, it said query will make a LEFT OUTER JOIN, I don’t know why this is different.

Second thing, now I want to get question objects in questions, I ran:

questions.each do |q|
  puts q.content
end

It returned error:

SELECT "questions".* FROM "questions" WHERE (id = 170,162,160) ORDER BY questions.created_at DESC
ActiveRecord::StatementInvalid: PG::Error: ERROR:  argument of WHERE must be type boolean, not type record
LINE 1: SELECT "questions".* FROM "questions"  WHERE (id = 170,162,1...

How can I get question objects now?

Answer 1

It looks like your where clause is wrong. Try:

where(:id => exam.question_ids)

When you provide the string version, “id = ?”, the database adapters won’t convert that to an in-clause. When you provide the hash version, the database adapters will recognize that the value is an array and use an in.

Perhaps a more efficient way would be to approach the problem differently:

class Question
  has_many :exam_questions
end

questions = Question.joins(:exam_questions).where(:exam_questions => {:exam_id => 16})

See Efficient ActiveRecord has_and_belongs_to_many query