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Suggest using preload to avoid N+1 cached query

Qingyu Zhao requested to merge document-suggest-preload-object-to-avoid-cached-n-plus-one-query into master

What does this MR do and why?

In the document https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/development/merge_request_performance_guidelines.md#cached-queries, we have suggested to

pipeline.builds.each do |build|
  build.project = pipeline.project
  build.to_json(only: [:name], include: [project: { only: [:name]}])
end

I think this is less intuitive, because it requires us to first assume build.project MUST be pipeline.project. I think the easier approach is to preload the required association:

# preload the required associations
ActiveRecord::Associations::Preloader.new.preload(pipeline, [builds: :project])

pipeline.builds.each do |build|
  build.to_json(only: [:name], include: [project: { only: [:name]}])
end

BTW: if we search in Gitlab source code, we could see there are many places already used ActiveRecord::Associations::Preloader.new.preload.

We could verity preload behaves as expected, refer to this comment.

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Edited by Qingyu Zhao

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