pluck.rb

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'
  gem 'rails', git: 'https://github.com/rails/rails'
  gem 'sqlite3'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

# 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 :posts, force: true  do |t|
  end

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

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_association_stuff
    3.times.each do
      post = Post.create
      5.times.each do
        post.comments << Comment.create!
      end
    end

    assert_equal 3,  Post.pluck(:id).count
    assert_equal 15, Post.joins(:comments).pluck(:id).count
    assert_equal 3,  Post.includes(:comments).pluck(:id).count # expect 3, not 15
  end
end