ruby 24 lines · 1 tab

N+1 prevention with includes and preload

Alex Kumar Jan 2026

1 tab

module Api
  module V1
    class PostsController < BaseController
      def index
        # Eager load author and recent comments with their authors
        posts = Post.published
                    .includes(:author, comments: [:author])
                    .order(created_at: :desc)
                    .page(params[:page])
                    .per(20)

        render json: posts, include: ['author', 'comments', 'comments.author']
      end

      def show
        # Eager load all associations needed for detail view
        post = Post.includes(:author, :tags, comments: [:author])
                   .find(params[:id])

        render json: post, include: ['author', 'tags', 'comments', 'comments.author']
      end
    end
  end
end

1 file · ruby Explain with highlit

Eager loading associations with includes, preload, or eager_load is essential for avoiding N+1 queries that kill performance. When rendering a list of posts with their authors, Post.includes(:author) loads all authors in a second query rather than firing one query per post. The difference between these methods matters: includes uses LEFT OUTER JOIN when you filter on associations, preload always uses separate queries, and eager_load forces a join. For nested associations like Post.includes(comments: :author), Rails handles the complexity of loading multiple levels efficiently. I verify eager loading works by checking the query count in logs or using Bullet gem to catch regressions.

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