class SampleSerializer(Serializer):
    user_id = serializers.PrimaryKeyRelatedField(queryset=User.objects.all())
    user_id_with_pk_field = serializers.PrimaryKeyRelatedField(queryset=User.objects.all(), pk_field=serializers.UUIDField())

# in docs
Sample:
  type: object
  properties:
    user_id:
      type: integer (or string)
    user_id_with_pk_field:
      type: string
      format: uuid