code: def to_db(self, value):
          
  
          # ensure value is valid
          self.validate(value)
  
          assert isinstance(value, list)
          value = list(value)
          for i, v in enumerate(value):
              value[i] = self.value_to_db(v)
  
          # return result
          assert isinstance(value, list)
          return value
docs: Returns field's single value prepared for saving into a database.