def singleton_belongs_to(*symbols, &block)
        options = symbols.extract_options!
        options.merge!(singleton: true)
        belongs_to(*symbols, options, &block)
      end
def singleton_belongs_to(*symbols, &block)
        options = symbols.extract_options!
        options.merge!(singleton: true)
        belongs_to(*symbols, options, &block)
      end
A quick method to declare singleton belongs to.