docs: def prepend_model(self, value, model):
          """
          Prepends model name if it is not already prepended.
          For example model is "Offer":
  
              key -> Offer.key
              -key -> -Offer.key
              Offer.key -> Offer.key
              -Offer.key -> -Offer.key
          """
          if '.' not in value:
              direction = ''
              if value.startswith('-'):
                  value = value[1:]
                  direction = '-'
              value = '%s%s.%s' % (direction, model, value)
          return value