Method _check_on_delete

django/db/models/fields/related.py:1073–1160

(self, databases)

            )

    def _check_on_delete(self, databases):
        on_delete = getattr(self.remote_field, "on_delete", None)
        errors = []
        if on_delete == DB_CASCADE:
            errors.extend(
                self._check_on_delete_db_support(
                    on_delete, "supports_on_delete_db_cascade", databases
                )
            )
        if on_delete == DB_SET_NULL:
            errors.extend(
                self._check_on_delete_db_support(
                    on_delete, "supports_on_delete_db_null", databases
                )
            )
        if on_delete in [DB_SET_NULL, SET_NULL] and not self.null:
            errors.append(
                checks.Error(
                    f"Field specifies on_delete={on_delete.__name__}, but cannot be "
                    "null.",
                    hint=(
                        "Set null=True argument on the field, or change the on_delete "
                        "rule."
                    ),
                    obj=self,
                    id="fields.E320",
                )
            )
        elif on_delete == SET_DEFAULT and not self.has_default():
            errors.append(
                checks.Error(
                    "Field specifies on_delete=SET_DEFAULT, but has no default value.",
                    hint="Set a default value, or change the on_delete rule.",
                    obj=self,
                    id="fields.E321",
                )
            )
        elif on_delete == DB_SET_DEFAULT:
            if self.db_default is NOT_PROVIDED:
                errors.append(
                    checks.Error(
                        "Field specifies on_delete=DB_SET_DEFAULT, but has "
                        "no db_default value.",
                        hint="Set a db_default value, or change the on_delete rule.",
                        obj=self,
                        id="fields.E322",
                    )
                )
            errors.extend(
                self._check_on_delete_db_support(
                    on_delete, "supports_on_delete_db_default", databases
                )
            )
        if not isinstance(self.remote_field.model, str) and on_delete != DO_NOTHING:
            # Database and Python variants cannot be mixed in a chain of
            # model references.
            is_db_on_delete = isinstance(on_delete, DatabaseOnDelete)
            ref_model_related_fields = (