code: def _get_model_fields(self, field_names, declared_fields, extra_kwargs):
          
          model = getattr(self.Meta, 'model')
          model_fields = {}
  
          for field_name in field_names:
              if field_name in declared_fields:
                  # If the field is declared on the serializer
                  field = declared_fields[field_name]
                  source = field.source or field_name
              else:
                  try:
                      source = extra_kwargs[field_name]['source']
                  except KeyError:
                      source = field_name
  
              if '.' in source or source == '*':
                  # Model fields will always have a simple source mapping,
                  # they can't be nested attribute lookups.
                  continue
  
              try:
                  field = model._meta.get_field(source)
                  if isinstance(field, DjangoModelField):
                      model_fields[source] = field
              except FieldDoesNotExist:
                  pass
  
          return model_fields
docs: Returns all the model fields that are being mapped to by fields
          on the serializer class.
          Returned as a dict of 'model field name' -> 'model field'.
          Used internally by `get_uniqueness_field_options`.