⚡️(backend) prefetch thread comments tree to fix N+1 queries

Retrieving a thread serialized its nested comments, reactions and
reaction users without prefetching, issuing one query per comment for
its author and reactions and one query per reaction for its users.
    
Signed-off-by: Mohamed El Amine BOUKERFA <boukerfa.ma@gmail.com>

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to
 ### Fixed
 
 - 🐛(frontend) overlap of block menu dropdown #2406
+- ⚡️(backend) fix N+1 queries when serializing thread comments #2415
 
 ## [v5.2.1] - 2026-06-05
 

src/backend/core/api/viewsets.py

@@ -3168,7 +3168,16 @@ class ThreadViewSet(
     permission_classes = [permissions.CommentPermission]
     pagination_class = None
     serializer_class = serializers.ThreadSerializer
-    queryset = models.Thread.objects.select_related("creator", "document")
+    queryset = models.Thread.objects.select_related(
+        "creator", "document"
+    ).prefetch_related(
+        db.Prefetch(
+            "comments",
+            queryset=models.Comment.objects.select_related("user").prefetch_related(
+                "reactions__users"
+            ),
+        ),
+    )
     resource_field_name = "document"
 
     def perform_create(self, serializer):

src/backend/core/tests/documents/test_api_documents_threads.py

@@ -512,6 +512,42 @@ def test_api_documents_threads_list_restricted_document_editor(role):
     assert len(response.json()) == 3
 
 
+@pytest.mark.parametrize("nb_threads", [1, 3])
+def test_api_documents_threads_list_number_of_queries(
+    nb_threads, django_assert_num_queries
+):
+    """
+    Listing threads should run a constant number of queries whatever the number
+    of threads, comments, reactions and reaction users.
+    """
+    document = factories.DocumentFactory(
+        link_reach="public",
+        link_role=models.LinkRoleChoices.COMMENTER,
+    )
+
+    for _ in range(nb_threads):
+        thread = factories.ThreadFactory(document=document)
+        for _ in range(2):
+            comment = factories.CommentFactory(thread=thread)
+            for emoji in ["👍", "🎉"]:
+                factories.ReactionFactory(
+                    comment=comment,
+                    emoji=emoji,
+                    users=factories.UserFactory.create_batch(2),
+                )
+
+    client = APIClient()
+    # 1 query for the document (permission check), 1 for the threads and 1 per
+    # prefetched relation: comments, reactions and reaction users.
+    with django_assert_num_queries(5):
+        response = client.get(
+            f"/api/v1.0/documents/{document.id!s}/threads/",
+        )
+
+    assert response.status_code == 200
+    assert len(response.json()) == nb_threads
+
+
 # Retrieve
 
 
@@ -857,6 +893,41 @@ def test_api_documents_threads_retrieve_restricted_document_privileged_roles(rol
     }
 
 
+@pytest.mark.parametrize("nb_comments", [1, 3])
+def test_api_documents_threads_retrieve_number_of_queries(
+    nb_comments, django_assert_num_queries
+):
+    """
+    Retrieving a thread should run a constant number of queries whatever the
+    number of comments, reactions and reaction users.
+    """
+    document = factories.DocumentFactory(
+        link_reach="public",
+        link_role=models.LinkRoleChoices.COMMENTER,
+    )
+
+    thread = factories.ThreadFactory(document=document)
+    for _ in range(nb_comments):
+        comment = factories.CommentFactory(thread=thread)
+        for emoji in ["👍", "🎉"]:
+            factories.ReactionFactory(
+                comment=comment,
+                emoji=emoji,
+                users=factories.UserFactory.create_batch(2),
+            )
+
+    client = APIClient()
+    # 1 query for the thread and 1 per prefetched relation: comments, reactions
+    # and reaction users.
+    with django_assert_num_queries(4):
+        response = client.get(
+            f"/api/v1.0/documents/{document.id!s}/threads/{thread.id!s}/",
+        )
+
+    assert response.status_code == 200
+    assert len(response.json()["comments"]) == nb_comments
+
+
 # Destroy