Problem with has_many :through counter_caches

bug.rb

require 'rubygems'
require 'active_record'

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

# Connect to an in-memory sqlite3 database
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|
    t.integer :projects_count, :default => 0
  end

  create_table :projects, :force => true do |t|
    t.integer :users_count, :default => 0
  end
  
  create_table :memberships, :force => true do |t|
    t.integer :user_id
    t.integer :project_id
  end
end

# define the models
class User < ActiveRecord::Base
  has_many :memberships
  has_many :projects, :through => :memberships
end

class Project < ActiveRecord::Base
  has_many :memberships
  has_many :users, :through => :memberships
end

class Membership < ActiveRecord::Base
  belongs_to :user
  belongs_to :project
end

### Here's the bugs:

user = User.create
project = Project.create

puts "Initially:"
puts "User's projects_count is #{user.projects_count}"
puts "Projects's users_count is #{project.users_count}"

user.projects << project

puts "After adding the project once:"
puts "User's projects_count is #{user.projects_count}"
puts "Projects's users_count is #{project.users_count}"

user.projects << project

puts "After adding the project a second time:"
puts "User's projects_count is #{user.projects_count}"
puts "Projects's users_count is #{project.users_count}"

Output is:

Active Record 3.1.3
-- create_table(:users, {:force=>true})
   -> 0.0417s
-- create_table(:projects, {:force=>true})
   -> 0.0005s
-- create_table(:memberships, {:force=>true})
   -> 0.0004s
Initially:
User's projects_count is 0
Projects's users_count is 0
After adding the project once:
User's projects_count is 1
Projects's users_count is 0
After adding the project a second time:
User's projects_count is 2
Projects's users_count is 0