class Project
  include Mongoid::Document

  field :title

  has_many :tasks, :dependent => :destroy

  accepts_nested_attributes_for :tasks, :allow_destroy => true
end

class Task
  include Mongoid::Document

  field :title

  belongs_to :project
end

class Log
  include Mongoid::Document

  field :message
end

class TaskObserver < Mongoid::Observer
  def after_save(task)
    Log.create!(:message => "Task created")
  end
end

>> p = Project.create!
>> p.tasks_attributes = [{:title => "My Task"}]
[
    [0] {
        :title => "My Task"
    }
]

>> Project.first.tasks.to_a
[
    [0] #<Task _id: 4ed61a11662e0c7ad400000d, title: "My Task", project_id: BSON::ObjectId('4ed6199a662e0c7ad4000007'), _type: nil>,
    [1] #<Task _id: 4ed62243662e0c7fc8000023, title: nil, project_id: nil, _type: nil, message: "Task created">
]

>> Log.all.to_a
[]