🐛(back) handle Django ValidationError as an accepted exception

When the django model raised a ValidationError, this exception is not
managed by DRF. To fix this issue we have to override the DRF
exception_handler.
The solution was found in this gist :
https://gist.github.com/twidi/9d55486c36b6a51bdcb05ce3a763e79f

CHANGELOG.md

@@ -12,6 +12,10 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 
 - Switch to QVBR rate control mode in live profiles
 
+### Fixed
+
+- Handle Django ValidationError as an accepted exception
+
 ### Removed
 
 - Dash endpoint in mediapackage channel

src/backend/marsha/core/tests/test_api_thumbnail.py

@@ -245,6 +245,28 @@ def test_api_thumbnail_create_instructor(self):
             },
         )
 
+    def test_api_thumbnail_create_already_existing_instructor(self):
+        """Creating a thumbnail should fail when a thumbnail already exists for the video."""
+        video = VideoFactory()
+        ThumbnailFactory(video=video)
+
+        jwt_token = AccessToken()
+        jwt_token.payload["resource_id"] = str(video.id)
+        jwt_token.payload["roles"] = [random.choice(["instructor", "administrator"])]
+        jwt_token.payload["permissions"] = {"can_update": True}
+
+        response = self.client.post(
+            "/api/thumbnails/", HTTP_AUTHORIZATION="Bearer {!s}".format(jwt_token)
+        )
+
+        self.assertEqual(response.status_code, 400)
+        content = json.loads(response.content)
+
+        self.assertEqual(
+            content,
+            {"video": ["Thumbnail with this Video already exists."]},
+        )
+
     def test_api_thumbnail_instructor_create_in_read_only(self):
         """Instructor should not be able to create thumbnails in a read_only mode."""
         thumbnail = ThumbnailFactory()

src/backend/marsha/core/views.py

@@ -6,7 +6,10 @@
 
 from django.conf import settings
 from django.core.cache import cache
-from django.core.exceptions import ImproperlyConfigured
+from django.core.exceptions import (
+    ImproperlyConfigured,
+    ValidationError as DjangoValidationError,
+)
 from django.templatetags.static import static
 from django.utils.decorators import method_decorator
 from django.views.decorators.clickjacking import xframe_options_exempt
@@ -15,6 +18,8 @@
 from django.views.generic.base import TemplateResponseMixin, TemplateView
 
 from pylti.common import LTIException
+from rest_framework.exceptions import ValidationError as DRFValidationError
+from rest_framework.views import exception_handler as drf_exception_handler
 from rest_framework_simplejwt.tokens import AccessToken
 from waffle import switch_is_active
 
@@ -30,6 +35,26 @@
 logger = getLogger(__name__)
 
 
+def exception_handler(exc, context):
+    """Handle Django ValidationError as an accepted exception.
+
+    For the parameters, see ``exception_handler``
+    This code comes from twidi's gist:
+    https://gist.github.com/twidi/9d55486c36b6a51bdcb05ce3a763e79f
+    """
+    if isinstance(exc, DjangoValidationError):
+        if hasattr(exc, "message_dict"):
+            detail = exc.message_dict
+        elif hasattr(exc, "message"):
+            detail = exc.message
+        elif hasattr(exc, "messages"):
+            detail = exc.messages
+
+        exc = DRFValidationError(detail=detail)
+
+    return drf_exception_handler(exc, context)
+
+
 @method_decorator(csrf_exempt, name="dispatch")
 @method_decorator(xframe_options_exempt, name="dispatch")
 class BaseView(ABC, TemplateResponseMixin, View):

src/backend/marsha/settings.py

@@ -152,7 +152,8 @@ class Base(Configuration):
     REST_FRAMEWORK = {
         "DEFAULT_AUTHENTICATION_CLASSES": (
             "rest_framework_simplejwt.authentication.JWTTokenUserAuthentication",
-        )
+        ),
+        "EXCEPTION_HANDLER": "marsha.core.views.exception_handler",
     }
 
     # WAFFLE