bug.txt

ruby-1.9.2-p180 :001 > p = Post.create
 => #<Post id: 1, title: nil, comments_count: 0, created_at: "2011-08-29 02:58:04", updated_at: "2011-08-29 02:58:04"> 
ruby-1.9.2-p180 :002 > p.comments.create
 => #<Comment id: 1, title: nil, post_id: 1, created_at: "2011-08-29 02:58:16", updated_at: "2011-08-29 02:58:16"> 
ruby-1.9.2-p180 :003 > p.comments.create
 => #<Comment id: 2, title: nil, post_id: 1, created_at: "2011-08-29 02:58:18", updated_at: "2011-08-29 02:58:18"> 
ruby-1.9.2-p180 :004 > p.reload
 => #<Post id: 1, title: nil, comments_count: 2, created_at: "2011-08-29 02:58:04", updated_at: "2011-08-29 02:58:04"> 
ruby-1.9.2-p180 :005 > Post.create
 => #<Post id: 2, title: nil, comments_count: 0, created_at: "2011-08-29 02:58:30", updated_at: "2011-08-29 02:58:30"> 
ruby-1.9.2-p180 :006 > c = Comment.last
 => #<Comment id: 2, title: nil, post_id: 1, created_at: "2011-08-29 02:58:18", updated_at: "2011-08-29 02:58:18"> 

# Omg the following updates counters without the record being saved!

ruby-1.9.2-p180 :007 > c.post = Post.last
 => #<Post id: 2, title: nil, comments_count: 0, created_at: "2011-08-29 02:58:30", updated_at: "2011-08-29 02:58:30"> 

# Omg notice the counts are off!

ruby-1.9.2-p180 :008 > p.reload
 => #<Post id: 1, title: nil, comments_count: 1, created_at: "2011-08-29 02:58:04", updated_at: "2011-08-29 02:58:04"> 
ruby-1.9.2-p180 :009 > p.comments.count
 => 2 

models.rb

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post, :counter_cache => true
end