mongoid_paranoia.rb

class Project
  include Mongoid::Document
  include Mongoid::Timestamps
  ....
  embeds_many :expenses
  ....  
end


class Expense
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::Paranoia
  field :amount, :type => Integer
  field :description
  field :expense_type, :type => Integer
  field :expense_date, :type => Date
  field :users_name  
  field :tags, :type => Array
  field :user_id

  index :tags
  index :user_id
  index(
    [
      [:expense_date, Mongo::DESCENDING],
      [:created_at, Mongo::DESCENDING]
    ])
  embedded_in :project, :inverse_of => :expenses
  ....
end