This demonstrates a bug in the Rails implicit counter_cache. When creating a record with nested records via a nested form, it does not increment the {child}s_count, but when deleting via a nested form, it decrements the field.

counter_cache_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 "photos", :force => true do |t|
    t.integer  "posting_id"
  end

  create_table "postings", :force => true do |t|
    t.integer  "photos_count", :default => 0
  end
end

# define the models
class Photo < ActiveRecord::Base
  belongs_to :posting
end

class Posting < ActiveRecord::Base
  has_many :photos
  accepts_nested_attributes_for :photos, :allow_destroy => true
end

### Here's the bugs:

posting = Posting.create(:photos_attributes => [Photo.new.attributes])

puts "Create posting with one photo via nested form:"
puts "Posting\#photos_count: #{posting.photos_count}; Expected: 1"

posting.update_attributes(:photos_attributes => [:id => posting.photos.first.id, :_destroy => true])

puts "Delete posting's photo via nested form:"
puts "Posting\#photos_count: #{posting.photos_count}; Expected: 0"