counter_cache destroy concurrency issues

You have the PostController, you are loading data into a variable to destroy it. For example two processes want to destroy the same post. One of them are getting data from the load_post and the second one got the same object of the post. They trying to destroy and no exception will be thrown, and the counter in topic will be decremented twice.

before_filter :load_post

def destroy
  @post.destroy
end

def load_post
  @post = Post.find_by_id(params[:id])
end

class Topic
  has_one :posts, :dependent => :destroy
end

class Post
  belongs_to :topic, :counter_cache => true
end

class CounterCacheConcurrencyTest < Test::Unit::TestCase
  def test_counter_cache_concurrency_test
    t = Topic.new

    posts_count_old = t.posts_count

    t.posts << Post.create



    p = t.posts.first
    p.destroy
    p.destroy

    # Will be failed, because destroy will invoke decrement_counter twice
    # and the actual value will be -1
    assert_equal posts_count_old, t.posts_count
  end
end

Could be fixed by adding an invoke of the after_filter, if destroy will affect at least one record in the database.