What is the efficient way of counting 2 level nested model (using mongoid or MongoDB in general)?

I have following data models. In which a Project embeds many ComponentDescriptor, and a ComponentDescriptor embeds many Statistic.

class Project
  include Mongoid::Document
  embeds_many :component_descriptors

  field :status, :type => Integer

  backgrounded :publish 
end

class ComponentDescriptor
  include Mongoid::Document
  include Mongoid::Acts::Tree

  embeds_many :statistics
  embedded_in :project, :inverse_of => :com开发者_JS百科ponent_descriptors

end


class Statistic
  include Mongoid::Document
  field :statistics_type, :type => String
  field :data, :type => String
  field :playhead_time, :type => String
  field :remote_ip, :type => String
  field :user_agent, :type => String

  embedded_in :component_descriptor, :inverse_of => :statistics
end

The question is what is the best way to count the total number of Statistic object in a Project.

One way I can think of is looping through each ComponentDescriptor and count the number of Statistics objects and then sum them up. But I think this is not efficient way.

Thank in advance.

If you can store the count field at the Project level that would be the most optimal and fastest way for you to obtain it.