ActiveRecord 5.1.3 Bug report: Joins does not populate association_cache for all records.

joins_association_cache_miss.rb

# frozen_string_literal: true

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  # Activate the gem you are reporting the issue against.
  gem "activerecord", "5.1.3"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :authors, force: :true do |t|
  end

  create_table :posts, force: true do |t|
    t.integer :author_id
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
  end
end

class Author < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  has_many :comments
  belongs_to :author
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_preload_association_cache_combined_with_joins
    first_author = Author.create!
    first_post = first_author.posts.create!
    first_post.comments.create!
    first_post.comments.create!

    second_author = Author.create!
    second_post = second_author.posts.create!
    second_post.comments.create!

    # The universe exists
    assert_equal 2, first_post.comments.count
    assert_equal 3, Comment.count
    assert_equal 2, Post.count
    assert_equal first_post.id, Comment.first.post.id
    assert_equal second_post.id, Comment.last.post.id

    # `post.author` is cached when no joins
    Post.preload(:author).each do |post|
      assert post.association_cached?(:author)
    end

    # Bug report: `post.author` is not cached when a join brings in
    # a "duplicated" post.
    posts = Post.joins(:comments).preload(:author)

    refute_equal posts.first.author, posts.last.author
    assert_equal posts.first.author, posts.second.author

    posts.each do |post|
      assert post.association_cached?(:author)
    end
  end
end