Nil Boolean Mongoid custom serialization with blank strings

gistfile1.rb

class TriBoolean

  attr_reader :value

  def initialize(value)
    @value = value
  end

  # Converts an object of this instance into a database friendly value.
  def mongoize
    return nil if boolean_blank?
    Boolean.mongoize(value)
  end

  class << self

    # Get the object as it was stored in the database, and instantiate
    # this custom class from it.
    def demongoize(object)
      object
    end

    # Takes any possible object and converts it to how it would be
    # stored in the database.
    def mongoize(object)
      TriBoolean.new(object).mongoize
    end

    # Converts the object that was supplied to a criteria and converts it
    # into a database friendly form.
    def evolve(object)
      # https://groups.google.com/forum/?fromgroups=#!topic/mongoid/78dPs0_GnWU
      mongoize(object)
    end
  end

  private

   def boolean_blank?
     !value.is_a?(Boolean) && value.blank?
   end
end

FWIW - I found that Boolean.mongoize was returning a string for some reason (which ended up always storing and returning a string from the database), so I ended up using Boolean.evolve in place of that, in the mongoize function, which correctly converts "true" to a Boolean true