# File lib/mongo_mapper/plugins/keys/key.rb, line 81
        def default_value
          return unless default?
          if default.instance_of? Proc
            type.to_mongo default.call
          else
            # Using Marshal is easiest way to get a copy of mutable objects
            # without getting an error on immutable objects
            type.to_mongo Marshal.load(Marshal.dump(default))
          end
        end