Fix AttributeError when using optimizer and prefetch_related (#533)

* Add a test capturing the failure

* Short-circuit when a queryset has been sliced

There are several places in the codebase where qs._result_cache is evaluated.

If a QuerySet has been sliced (e.g., prefetched with pagination in DjangoOptimizerExtension), it has been evaluated as a list and will not have the `_result_cache` attrib.

* Update test to verify short-circuiting works as expected

strawberry_django/optimizer.py

@@ -694,6 +694,10 @@ def optimize(
     if isinstance(qs, BaseManager):
         qs = cast(QuerySet[_M], qs.all())
 
+    if isinstance(qs, list):
+        # return sliced queryset as-is
+        return qs
+
     # Avoid optimizing twice and also modify an already resolved queryset
     if (
         get_queryset_config(qs).optimized or qs._result_cache is not None  # type: ignore

strawberry_django/permissions.py

@@ -108,6 +108,10 @@ def filter_with_perms(qs: QuerySet[_M], info: Info) -> QuerySet[_M]:
     if not context.checkers or context.is_safe:
         return qs
 
+    if isinstance(qs, list):
+        # return sliced queryset as-is
+        return qs
+
     # Do not do anything is results are cached
     if qs._result_cache is not None:  # type: ignore
         set_perm_safe(False)

strawberry_django/resolvers.py

@@ -27,6 +27,10 @@
 
 
 def default_qs_hook(qs: models.QuerySet[_M]) -> models.QuerySet[_M]:
+    if isinstance(qs, list):
+        # return sliced queryset as-is
+        return qs
+
     # FIXME: We probably won't need this anymore when we can use graphql-core 3.3.0+
     # as its `complete_list_value` gives a preference to async iteration it if is
     # provided by the object.

tests/test_optimizer.py

@@ -1025,3 +1025,82 @@ def test_query_nested_connection_with_filter(db, gql_client: GraphQLTestClient):
     assert {
         edge["node"]["id"] for edge in result["issuesWithFilters"]["edges"]
     } == expected
+
+
+@pytest.mark.django_db(transaction=True)
+def test_query_with_optimizer_paginated_prefetch():
+    @strawberry_django.type(Milestone, pagination=True)
+    class MilestoneTypeWithNestedPrefetch:
+        @strawberry_django.field()
+        def name(self, info) -> str:
+            return self.name
+
+    @strawberry_django.type(
+        Project,
+    )
+    class ProjectTypeWithPrefetch:
+        @strawberry_django.field()
+        def name(self, info) -> str:
+            return self.name
+
+        milestones: List[MilestoneTypeWithNestedPrefetch]
+
+    milestone1 = MilestoneFactory.create()
+    project = milestone1.project
+    MilestoneFactory.create(project=project)
+
+    @strawberry.type
+    class Query:
+        projects: List[ProjectTypeWithPrefetch] = strawberry_django.field()
+
+    query1 = utils.generate_query(Query, enable_optimizer=False)
+    query_str = """
+      query TestQuery {
+        projects {
+            name
+            milestones (pagination: {limit: 1}) {
+              name
+            }
+        }
+      }
+    """
+
+    # NOTE: The following assertion doesn't work because the
+    # DjangoOptimizerExtension instance is not the one within the
+    # generate_query wrapper
+    """
+    assert DjangoOptimizerExtension.enabled.get()
+    """
+    result1 = query1(query_str)
+
+    assert isinstance(result1, ExecutionResult)
+    assert not result1.errors
+    assert result1.data == {
+        "projects": [
+            {
+                "name": project.name,
+                "milestones": [
+                    {
+                        "name": milestone1.name,
+                    },
+                ],
+            },
+        ],
+    }
+
+    query2 = utils.generate_query(Query, enable_optimizer=True)
+    result2 = query2(query_str)
+
+    assert isinstance(result2, ExecutionResult)
+    assert result2.data == {
+        "projects": [
+            {
+                "name": project.name,
+                "milestones": [
+                    {
+                        "name": milestone1.name,
+                    },
+                ],
+            },
+        ],
+    }