id summary reporter owner description type status component version severity resolution keywords cc stage has_patch needs_docs needs_tests needs_better_patch easy ui_ux 29655 Cannot enter Django admin interface when model instances are validated on save Evgeny Arshinov nobody "Hello, In #29549 I was advised to enable model validation on save to properly validate choice fields, like this: {{{ @receiver(pre_save) def pre_save_handler(sender, instance, *args, **kwargs): instance.full_clean() }}} Turned out this approach breaks Django session's middleware and, as a result, no one can enter Django admin interface (which uses the session middleware). Scenario: 1. Use a project that includes Django admin interface. 2. Enable model validation: {{{ from django.db.models.signals import pre_save from django.dispatch import receiver @receiver(pre_save) def pre_save_handler(sender, instance, *args, **kwargs): instance.full_clean() }}} 3. Visit Django admin interface start page (`/admin`). 4. Enter valid credentials. The result is error 500 due to a `ValidationError`: {{{ Internal Server Error: /admin/login/ Traceback (most recent call last): File ""C:\django_sample\venv\lib\site-packages\django\core\handlers\exception.py"", line 34, in inner response = get_response(request) File ""C:\django_sample\venv\lib\site-packages\django\utils\deprecation.py"", line 93, in __call__ response = self.process_response(request, response) File ""C:\django_sample\venv\lib\site-packages\django\contrib\sessions\middleware.py"", line 58, in process_response request.session.save() File ""C:\django_sample\venv\lib\site-packages\django\contrib\sessions\backends\db.py"", line 87, in save obj.save(force_insert=must_create, force_update=not must_create, using=using) File ""C:\django_sample\venv\lib\site-packages\django\db\models\base.py"", line 717, in save force_update=force_update, update_fields=update_fields) File ""C:\django_sample\venv\lib\site-packages\django\db\models\base.py"", line 742, in save_base update_fields=update_fields, File ""C:\django_sample\venv\lib\site-packages\django\dispatch\dispatcher.py"", line 175, in send for receiver in self._live_receivers(sender) File ""C:\django_sample\venv\lib\site-packages\django\dispatch\dispatcher.py"", line 175, in for receiver in self._live_receivers(sender) File ""C:\django_sample\django_sample\models.py"", line 41, in pre_save_handler instance.full_clean() File ""C:\django_sample\venv\lib\site-packages\django\db\models\base.py"", line 1151, in full_clean raise ValidationError(errors) django.core.exceptions.ValidationError: {'session_key': ['Session with this Session key already exists.']} }}} We dug this problem a bit, and found out that during the login procedure saving a `django.contrib.sessions.Session` instance is performed twice with the same session key: - In the first case, the object is saved with `force_insert=True, force_update=False` - In the second case, the object is saved with `force_insert=False, force_update=True` - Hovewer, in both cases the object's `_state.adding` flag is `True`, which causes `django.db.Model._perform_unique_checks` to include the value of the unique field of the object being saved (in this case, a session key) in unique validation. Since there is already an object with the given session key in the database after the first save, `validate_unique` reports an error. Traceback for the first save operation (some bottom frames omitted for brevity): {{{ ... File ""C:\django_sample\venv\lib\site-packages\django\contrib\auth\views.py"", line 90, in form_valid auth_login(self.request, form.get_user()) File ""C:\django_sample\venv\lib\site-packages\django\contrib\auth\__init__.py"", line 108, in login request.session.cycle_key() File ""C:\django_sample\venv\lib\site-packages\django\contrib\sessions\backends\base.py"", line 298, in cycle_key self.create() File ""C:\django_sample\venv\lib\site-packages\django\contrib\sessions\backends\db.py"", line 55, in create self.save(must_create=True) File ""C:\django_sample\venv\lib\site-packages\django\contrib\sessions\backends\db.py"", line 87, in save obj.save(force_insert=must_create, force_update=not must_create, using=using) File ""C:\django_sample\venv\lib\site-packages\django\db\models\base.py"", line 717, in save force_update=force_update, update_fields=update_fields) File ""C:\django_sample\venv\lib\site-packages\django\db\models\base.py"", line 742, in save_base update_fields=update_fields, File ""C:\django_sample\venv\lib\site-packages\django\dispatch\dispatcher.py"", line 175, in send for receiver in self._live_receivers(sender) File ""C:\django_sample\venv\lib\site-packages\django\dispatch\dispatcher.py"", line 175, in for receiver in self._live_receivers(sender) File ""C:\django_sample\django_sample\models.py"", line 41, in pre_save_handler instance.full_clean() }}} Traceback for the second save operation: {{{ ... File ""C:\django_sample\venv\lib\site-packages\django\contrib\sessions\middleware.py"", line 58, in process_response request.session.save() File ""C:\django_sample\venv\lib\site-packages\django\contrib\sessions\backends\db.py"", line 87, in save obj.save(force_insert=must_create, force_update=not must_create, using=using) File ""C:\django_sample\venv\lib\site-packages\django\db\models\base.py"", line 717, in save force_update=force_update, update_fields=update_fields) File ""C:\django_sample\venv\lib\site-packages\django\db\models\base.py"", line 742, in save_base update_fields=update_fields, File ""C:\django_sample\venv\lib\site-packages\django\dispatch\dispatcher.py"", line 175, in send for receiver in self._live_receivers(sender) File ""C:\django_sample\venv\lib\site-packages\django\dispatch\dispatcher.py"", line 175, in for receiver in self._live_receivers(sender) File ""C:\django_sample\django_sample\models.py"", line 41, in pre_save_handler instance.full_clean() }}}" Bug closed Uncategorized 2.1 Normal invalid Unreviewed 0 0 0 0 0 0