gistfile1.rb

class User < ActiveRecord::Base
  has_many :cars
end
class Car < ActiveRecord::Base
  belongs_to :user, :counter_cache => true
end

gistfile2.rb

user = User.create
=> #<User id: 1, cars_count: 0, created_at: "2008-01-14 20:40:14", updated_at: "2008-01-14 20:41:52">
car = Car.create
=> #<Car id: 1, user_id: nil, created_at: "2008-01-14 20:40:24", updated_at: "2008-01-14 20:41:50">
>> user.cars.size
=> 0
>> car.user = user
=> #<User id: 1, cars_count: 0, created_at: "2008-01-14 20:40:14", updated_at: "2008-01-14 20:41:52">
>> user
=> #<User id: 1, cars_count: 0, created_at: "2008-01-14 20:40:14", updated_at: "2008-01-14 20:41:52">
# First issue: users cars_count does not get updated in memory.