mongoid before_create.rb

require 'rubygems'
require 'mongoid'

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

Mongoid.master.collection("topics").drop
Mongoid.master.collection("members").drop
Mongoid.master.collection("posts").drop

class Topic
  include Mongoid::Document
  field :title
  field :creator

  embeds_many :members
  embeds_many :posts

  before_create :creator_as_members

  protected

  def creator_as_members
    members << Member.new(:user => creator)
  end
end

class Member
  include Mongoid::Document

  field :user
  field :unread, :type => Integer, :default => 0

  embedded_in :topic, :inverse_of => :members

  validates_uniqueness_of :user
end

class Post
  include Mongoid::Document

  field :nickname

  embedded_in :topics, :inverse_of => :posts

  before_create :set_unread

  protected

  def set_unread
    self.topics.members.each do |member|
      member.unread += 1
    end
  end

end

@topic = Topic.create(:title => "Test", :creator => "chatgris")

Topic.where(:title => "Test").first.members.each do |member|
  p member
end

p "----------------------"

@topic.posts.create(:nickname => "chatgris")
@topic.save

Topic.where(:title => "Test").first.members.each do |member|
  p member
end