I just spent more than hour debugging an odd issue with an embedded document's after_delete callback where it wasn't
working properly when calling destroy_all on the collection, but did work when calling each(&:destroy).

It's hard to explain without an example, so here's what I used:

<pre># Gemfile is using mongoid's Git HEAD
require 'bundler/setup'
require 'mongoid'

Mongoid.configure do |config|
config.master = Mongo::Connection.new.db('test')
end

class User
include Mongoid::Document

field :name, type: String

embeds_many :favorites
end

class Favorite
include Mongoid::Document

embedded_in :user

field :name, type: String

after_destroy :decrement_count
after_create :increment_count

protected

def decrement_count
FavoriteCount.decrement(self.name)
end

def increment_count
FavoriteCount.increment(self.name)
end
end

class FavoriteCount
include Mongoid::Document

field :name, type: String
field :count, type: Integer, default: 0

def self.decrement(name)
self.update_count(name, -1)
end

def self.increment(name)
self.update_count(name, 1)
end

protected

def self.update_count(name, value)
if fc = self.find_or_initialize_by(name: name)
fc.update_attribute(:count, fc.count + value)
end
end
end

if $0 == FILE
require 'rspec'

describe FavoriteCount do
after do
[User, FavoriteCount].each(&:destroy_all)
end

before do
3.times do |i|
user = User.create(name: "user#

{i}

")
user.favorites.create(name: 'mongodb')
end
end

it "updates when calling destroy_all" do
FavoriteCount.where(name: 'mongodb').first.count.should eql(3)
User.last.favorites.destroy_all

1. THIS FAILS; expected 2, got 3
   FavoriteCount.where(name: 'mongodb').first.count.should eql(2)
   end

it "updates when calling each(&:destroy)" do
FavoriteCount.where(name: 'mongodb').first.count.should eql(3)
User.last.favorites.each(&:destroy)

1. THIS PASSES
   FavoriteCount.where(name: 'mongodb').first.count.should eql(2)
   end
   end
   end</pre>

Both times the callback is being executed, but for some reason only the second way actually has the desired outcome. The only thing I can think of is some kind of weird race condition.