input.rb

gem 'rails', '3.2.13' # change as required
require 'active_record'

# Print out what version we're running
puts "Active Record #{ActiveRecord::VERSION::STRING}"

# Connect to an in-memory sqlite3 database (more on this in a moment)
ActiveRecord::Base.establish_connection(
  :adapter  => 'sqlite3',
  :database => ':memory:'
)

# Create the minimal database schema necessary to reproduce the bug
ActiveRecord::Schema.define do
  create_table :users, :force => true do |t|
  end

  create_table :user_folders, :force => true do |t|
    t.integer :user_id
  end
end

# Create the minimal set of models to reproduce the bug
class User < ActiveRecord::Base
  has_many :user_folders
end

class UserFolder < ActiveRecord::Base
  belongs_to :user
end

# Create some test data
#
# If you're demonstrating an exception, then this is probably not necessary,
# but if your bug is to do with the wrong data being returned from the database,
# then you'll probably need some test data to show that.
user = User.create!
user_folder = UserFolder.create(:user => user)

# Reproduce the actual bug!
user = User.first
puts "initial assocation is not loaded: #{user.user_folders.loaded?}"

puts "loading first_folder object out with 'user.user_folders.first'"
first_folder = user.user_folders.first
puts "user.user_folders.loaded? - #{user.user_folders.loaded?}"

puts "getting the first_folder_again via 'user.user_folders.first'"
first_folder_again = user.user_folders.first
puts "user.user_folders.loaded? - #{user.user_folders.loaded?}"

puts "first_folder == first_folder_again - #{first_folder == first_folder_again}"
puts "first_folder.object_id == first_folder_again.object_id - #{first_folder.object_id == first_folder_again.object_id}"
puts "user.user_folders.loaded?: #{user.user_folders.loaded?}"

puts "\n\nSTARTING OVER BUT LOADING THE ASSOCIATION FIRST\n\n"
user = User.first
puts "loading initial user_folders";
user.user_folders[0]
puts "initial assocation is not loaded: #{user.user_folders.loaded?}"

puts "loading first_folder object out with 'user.user_folders.first'"
first_folder = user.user_folders.first
puts "user.user_folders.loaded? - #{user.user_folders.loaded?}"

puts "getting the first_folder_again via 'user.user_folders.first'"
first_folder_again = user.user_folders.first
puts "user.user_folders.loaded? - #{user.user_folders.loaded?}"

puts "first_folder == first_folder_again - #{first_folder == first_folder_again}"
puts "first_folder.object_id == first_folder_again.object_id - #{first_folder.object_id == first_folder_again.object_id}"

output.txt

Active Record 3.2.13
-- create_table(:users, {:force=>true})
   -> 0.0137s
-- create_table(:user_folders, {:force=>true})
   -> 0.0005s
initial assocation is not loaded: false
loading first_folder object out with 'user.user_folders.first'
user.user_folders.loaded? - false
getting the first_folder_again via 'user.user_folders.first'
user.user_folders.loaded? - false
first_folder == first_folder_again - true
first_folder.object_id == first_folder_again.object_id - false
user.user_folders.loaded?: false


STARTING OVER BUT LOADING THE ASSOCIATION FIRST

loading initial user_folders
initial assocation is not loaded: true
loading first_folder object out with 'user.user_folders.first'
user.user_folders.loaded? - true
getting the first_folder_again via 'user.user_folders.first'
user.user_folders.loaded? - true
first_folder == first_folder_again - true
first_folder.object_id == first_folder_again.object_id - true