code: def serialize(self, obj, fields):
          
  
          data = {}
          remaining_fields = []
          for field in fields:
              if callable(field):  # Callable
                  data[field.__name__] = field(obj)
              elif hasattr(self, field) and callable(getattr(self, field)):  # Method on the view
                  data[field] = getattr(self, field)(obj)
              elif hasattr(obj, field):  # Callable/property/field on the model
                  attr = getattr(obj, field)
                  if isinstance(attr, Model):
                      data[field] = attr.pk
                  elif isinstance(attr, Manager):
                      data[field] = [item['pk'] for item in attr.values('pk')]
                  elif callable(attr):  # Callable on the model
                      data[field] = attr()
                  else:
                      remaining_fields.append(field)
              else:
                  raise AttributeError('Invalid field: %s' % field)
  
          # Add on db fields
          serializer = Serializer()
          serializer.serialize([obj], fields=list(remaining_fields))
          data.update(serializer.getvalue()[0]['fields'])
  
          # Any remaining fields should be properties on the model
          remaining_fields = set(remaining_fields) - set(data.keys())
  
          for field in remaining_fields:
              data[field] = getattr(obj, field)
  
          return data
docs: Serializes a single model instance to a Python dict, based on the specified list of fields.