Eager loading join causing n+1-1

n_plus_1_minus_1_setup.rb

# Create rails app to see issue.
rails new sample

rails g scaffold Author name:string
rails g scaffold Post title:string body:text author:references
rails g scaffold Comment body:text post:references

# Update post model to add association.
has_many :comments

# Update seed.rb
authors = Author.create([{name: 'User 1'}, {name: 'User 2'}])
posts = Post.create([{title: 'post 1', body: 'body of post 1', author: authors.first}, {title: 'post 2', body: 'body of post 2', author: authors.first}, {title: 'post 3', body: 'body of post 3', author: authors.second}])
comments = Comment.create([{body: 'comment 1', post: posts.first}, {body: 'comment 2', post: posts.first}, {body: 'comment 3', post: posts.first}, {body: 'comment 4', post: posts.second}, {body: 'comment 5', post: posts.third}, {body: 'comment 6', post: posts.third}, {body: 'comment 7', post: posts.third}, {body: 'comment 8', post: posts.third}])

# Run migrations and seed database.
rake db:migrate
rake db:seed

# Go to console to test output.
rails c

posts = Post.joins(:comments).select("posts.*, comments.body").includes(:author)
posts.each do |x|
  puts "#{x.title} #{x.author.name} #{x.body}"
end

# Console output.

# post 1 User 1 comment 1
#   Author Load (0.3ms)  SELECT  "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT 1  [["id", 1]]
# post 1 User 1 comment 2
#   Author Load (0.1ms)  SELECT  "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT 1  [["id", 1]]
# post 1 User 1 comment 3
# post 2 User 1 comment 4
# post 3 User 2 comment 5
#   Author Load (0.1ms)  SELECT  "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT 1  [["id", 2]]
# post 3 User 2 comment 6
#   Author Load (0.1ms)  SELECT  "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT 1  [["id", 2]]
# post 3 User 2 comment 7
#   Author Load (0.1ms)  SELECT  "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT 1  [["id", 2]]
# post 3 User 2 comment 8

posts = Post.joins(:comments).select("posts.*, comments.body").includes(:author)
posts.each do |x|
  puts x.association(:author).loaded?
end

# Output.

# true
# false
# false
# true
# true
# false
# false
# false


# Interestingly the following test does not fail.

class PostTest < ActiveSupport::TestCase
  test "should eager load author for each post" do 
    posts = Post.joins(:comments).select("posts.*, comments.body").includes(:author)
    posts.each do |x|
      assert x.association(:author).loaded?
      # Also assert x.author.loaded? passes but errors in the console.
    end
  end
end

# Test output.

# Running via Spring preloader in process 2614
# Run options: --seed 29018

# # Running:

# ......................

# Finished in 1.561278s, 14.0910 runs/s, 24.9795 assertions/s.

# 22 runs, 39 assertions, 0 failures, 0 errors, 0 skips


##############################################################

# This only eager loads the Author for the first instance of each post. 
# Each consecutive instance of a post triggers an additional query to find their authors.