Fix n+1 query issue in message deleteion context

pydis_site/apps/api/serializers.py

@@ -111,9 +111,10 @@ class DeletedMessageSerializer(ModelSerializer):
     model for more information.
     """
 
-    author = PrimaryKeyRelatedField(
-        queryset=User.objects.all()
-    )
+    # Plain integers to avoid per-row DB validators (id's UniqueValidator, author's
+    # FK lookup) that cause N+1 queries. Validation is done in the `MessageDeletionContextSerializer` instead.
+    id = IntegerField()
+    author = IntegerField(source='author_id', allow_null=True)
     deletion_context = PrimaryKeyRelatedField(
         queryset=MessageDeletionContext.objects.all(),
         # This will be overridden in the `create` function
@@ -146,6 +147,39 @@ class Meta:
         fields = ('actor', 'creation', 'id', 'deletedmessage_set')
         depth = 1
 
+    def validate_deletedmessage_set(self, messages: list[dict]) -> list[dict]:
+        """
+        Validate the deleted messages using batched queries to avoid N+1 lookups.
+
+        Rather than letting each nested message trigger its own author and
+        uniqueness queries, all referenced authors and message IDs are checked
+        with a single query each.
+        """
+        author_ids = {
+            message['author_id']
+            for message in messages
+            if message['author_id'] is not None
+        }
+        known_author_ids = set(
+            User.objects.filter(id__in=author_ids).values_list('id', flat=True)
+        )
+        unknown_author_ids = author_ids - known_author_ids
+        if unknown_author_ids:
+            raise ValidationError(
+                f"The following message authors do not exist: {sorted(unknown_author_ids)}"
+            )
+
+        message_ids = [message['id'] for message in messages]
+        existing_ids = set(
+            DeletedMessage.objects.filter(id__in=message_ids).values_list('id', flat=True)
+        )
+        if existing_ids:
+            raise ValidationError(
+                f"The following message IDs already exist: {sorted(existing_ids)}"
+            )
+
+        return messages
+
     def create(self, validated_data: dict) -> MessageDeletionContext:
         """
         Return a `MessageDeletionContext` based on the given data.

pydis_site/apps/api/tests/test_deleted_messages.py

@@ -1,5 +1,7 @@
 from datetime import UTC, datetime
 
+from django.db import connection
+from django.test.utils import CaptureQueriesContext
 from django.urls import reverse
 
 from .base import AuthenticatedAPITestCase
@@ -46,6 +48,91 @@ def test_accepts_valid_data(self):
         self.assertIsNone(context.actor)
 
 
+class DeletedMessagesQueryCountTests(AuthenticatedAPITestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.author = User.objects.create(
+            id=77,
+            name='Sportacus',
+            discriminator=77,
+        )
+
+    def _build_data(self, message_count: int) -> dict:
+        return {
+            'actor': None,
+            'creation': datetime.now(tz=UTC).isoformat(),
+            'deletedmessage_set': [
+                {
+                    'author': self.author.id,
+                    'id': 1000 + index,
+                    'channel_id': 5555,
+                    'content': f"Message {index}",
+                    'embeds': [],
+                    'attachments': []
+                }
+                for index in range(message_count)
+            ]
+        }
+
+    def test_query_count_is_independent_of_message_count(self):
+        """The number of queries must not grow with the size of the deletedmessage_set."""
+        url = reverse('api:bot:messagedeletioncontext-list')
+
+        with CaptureQueriesContext(connection) as few_ctx:
+            response = self.client.post(url, data=self._build_data(2))
+        self.assertEqual(response.status_code, 201)
+
+        MessageDeletionContext.objects.all().delete()
+
+        with CaptureQueriesContext(connection) as many_ctx:
+            response = self.client.post(url, data=self._build_data(20))
+        self.assertEqual(response.status_code, 201)
+
+        self.assertEqual(len(few_ctx.captured_queries), len(many_ctx.captured_queries))
+
+
+class DeletedMessagesValidationTests(AuthenticatedAPITestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.author = User.objects.create(
+            id=88,
+            name='Stephanie',
+            discriminator=88,
+        )
+
+    def _message(self, message_id: int, author_id: int) -> dict:
+        return {
+            'author': author_id,
+            'id': message_id,
+            'channel_id': 5555,
+            'content': "Content",
+            'embeds': [],
+            'attachments': []
+        }
+
+    def test_rejects_unknown_author(self):
+        url = reverse('api:bot:messagedeletioncontext-list')
+        data = {
+            'actor': None,
+            'creation': datetime.now(tz=UTC).isoformat(),
+            'deletedmessage_set': [self._message(1, author_id=999999)],
+        }
+        response = self.client.post(url, data=data)
+        self.assertEqual(response.status_code, 400)
+
+    def test_rejects_already_existing_message_id(self):
+        url = reverse('api:bot:messagedeletioncontext-list')
+        first = {
+            'actor': None,
+            'creation': datetime.now(tz=UTC).isoformat(),
+            'deletedmessage_set': [self._message(2, author_id=self.author.id)],
+        }
+        self.assertEqual(self.client.post(url, data=first).status_code, 201)
+
+        response = self.client.post(url, data=first)
+        self.assertEqual(response.status_code, 400)
+
+
 class DeletedMessagesWithActorTests(AuthenticatedAPITestCase):
     @classmethod
     def setUpTestData(cls):