Method _prepare_related_fields_for_save

django/db/models/base.py:1260–1312

(self, operation_name, fields=None)

            self.__dict__.pop(field.attname, None)

    def _prepare_related_fields_for_save(self, operation_name, fields=None):
        # Ensure that a model instance without a PK hasn't been assigned to
        # a ForeignKey, GenericForeignKey or OneToOneField on this model. If
        # the field is nullable, allowing the save would result in silent data
        # loss.
        for field in self._meta.concrete_fields:
            if fields and field not in fields:
                continue
            # If the related field isn't cached, then an instance hasn't been
            # assigned and there's no need to worry about this check.
            if field.is_relation and field.is_cached(self):
                obj = getattr(self, field.name, None)
                if not obj:
                    continue
                # A pk may have been assigned manually to a model instance not
                # saved to the database (or auto-generated in a case like
                # UUIDField), but we allow the save to proceed and rely on the
                # database to raise an IntegrityError if applicable. If
                # constraints aren't supported by the database, there's the
                # unavoidable risk of data corruption.
                if not obj._is_pk_set():
                    # Remove the object from a related instance cache.
                    if not field.remote_field.multiple:
                        field.remote_field.delete_cached_value(obj)
                    raise ValueError(
                        "%s() prohibited to prevent data loss due to unsaved "
                        "related object '%s'." % (operation_name, field.name)
                    )
                elif getattr(self, field.attname) in field.empty_values:
                    # Set related object if it has been saved after an
                    # assignment.
                    setattr(self, field.name, obj)
                # If the relationship's pk/to_field was changed, clear the
                # cached relationship.
                if getattr(obj, field.target_field.attname) != getattr(
                    self, field.attname
                ):
                    field.delete_cached_value(self)
        # GenericForeignKeys are private.
        for field in self._meta.private_fields:
            if fields and field not in fields:
                continue
            if (
                field.is_relation
                and field.is_cached(self)
                and hasattr(field, "fk_field")
            ):
                obj = field.get_cached_value(self, default=None)
                if obj and not obj._is_pk_set():
                    raise ValueError(
                        f"{operation_name}() prohibited to prevent data loss due to "
                        f"unsaved related object '{field.name}'."
                    )

    def delete(self, using=None, keep_parents=False):
        if not self._is_pk_set():