N+1 issue when updating a model with a relationship

Question

I have a N+1 problem which I cannot solve. I have a sample app hosted here that shows the issue: https://github.com/davidmles/question_parts

This is how it works:

There are 4 models: question, question_part, answer, and answer_part. A question has N parts, and also N answers. An answer has N parts. And an answer part belongs to an answer and to a question part (the one is answering).

An answer can be a draft or published. A draft does not need to have all its parts answered, while a published answer does need them answered.

The problem:

When an answer is created, but not all its parts are answered, and I try to publish it, validation will fail and the edit form will be shown again. When showing that edit form after validation failed, there will be a N+1 problem, as answer parts will load the corresponding question parts without eager loading them.

I have tried to eager load them, but then the edit form will not show the proper error.

How could I solve this? I have added more information in app's readme file, as well as steps to guide on seeing the problem: https://github.com/davidmles/question_parts/blob/master/README.md

Answer 1

Bad news: it seems that ActiveRecord does not play nicely with #includes in collection associations. And actually you cannot hack it because it does not persist relation object ( here ).

Good news: I found some workaround.

Add has_many :question_parts, through: :parts to your Answer model. After that use this finder in answers_controller: @answer = Answer.preload(:question_parts).where(id: params[:id]).first . Occasionally bullet gem does not like that (it says unused eager loading blabla but it's not true) but there is no N + 1 query and the form works fine.

Answer 2

Just to complement Flash Gordon's answer, the query could also be written like this:

@answer = Answer.preload(parts: [:question_part]).where(id: id).first

So, no modification to Answer is required.

Also, using includes in place of preload seems to work (Rails chooses the right strategy in this case).