Deduce model field names from custom prefetches

strawberry_django/optimizer.py

@@ -189,6 +189,17 @@ def with_hints(
             ),
         )
 
+    def get_custom_prefetches(self, info: GraphQLResolveInfo) -> list[Prefetch]:
+        custom_prefetches = []
+        for p in self.prefetch_related:
+            if isinstance(p, Callable):
+                assert_type(p, PrefetchCallable)
+                p = p(info)  # noqa: PLW2901
+
+            if isinstance(p, Prefetch) and p.to_attr is not None:
+                custom_prefetches.append(p)
+        return custom_prefetches
+
     def with_prefix(self, prefix: str, *, info: GraphQLResolveInfo):
         prefetch_related = []
         for p in self.prefetch_related:
@@ -458,7 +469,8 @@ def _get_model_hints(
             continue
 
         # Add annotations from the field if they exist
-        field_store = getattr(field, "store", None)
+        field_store = cast(OptimizerStore | None, getattr(field, "store", None))
+        custom_prefetches: list[Prefetch] = []
         if field_store is not None:
             if (
                 len(field_store.annotate) == 1
@@ -477,20 +489,45 @@ def _get_model_hints(
             store |= (
                 field_store.with_prefix(prefix, info=info) if prefix else field_store
             )
+            custom_prefetches = field_store.get_custom_prefetches(info)
 
         # Then from the model property if one is defined
         model_attr = getattr(model, field.python_name, None)
         if model_attr is not None and isinstance(model_attr, ModelProperty):
             attr_store = model_attr.store
             store |= attr_store.with_prefix(prefix, info=info) if prefix else attr_store
+            attr_store_prefetches = attr_store.get_custom_prefetches(info)
+            if attr_store_prefetches:
+                custom_prefetches.extend(attr_store_prefetches)
+
+        model_fieldname: str | None = None
+        model_field = None
+        # try to find the model field name in any custom prefetches
+        if custom_prefetches:
+            for prefetch in custom_prefetches:
+                prefetch_field = model_fields.get(prefetch.prefetch_through, None)
+                if prefetch_field:
+                    if not model_field:
+                        model_field = prefetch_field
+                        model_fieldname = prefetch.prefetch_through
+                    elif model_field != prefetch_field:
+                        # we found more than one model field from the custom prefetches
+                        # not much we can do here
+                        model_field = None
+                        model_fieldname = None
+                        custom_prefetches = []
+                        break
 
         # Lastly, from the django field itself
-        model_fieldname: str = getattr(field, "django_name", None) or field.python_name
-        model_field = model_fields.get(model_fieldname, None)
+        if not model_fieldname:
+            model_fieldname = getattr(field, "django_name", None) or field.python_name
+            model_field = model_fields.get(model_fieldname, None)
+
         if model_field is not None:
             path = f"{prefix}{model_fieldname}"
 
-            if isinstance(model_field, (models.ForeignKey, OneToOneRel)):
+            if not custom_prefetches and isinstance(model_field, (models.ForeignKey, OneToOneRel)):
+                # only select_related if there is no custom prefetch
                 store.only.append(path)
                 store.select_related.append(path)
 
@@ -517,7 +554,7 @@ def _get_model_hints(
                     if f_store is not None:
                         cache.setdefault(f_model, []).append((level, f_store))
                         store |= f_store.with_prefix(path, info=info)
-            elif GenericForeignKey and isinstance(model_field, GenericForeignKey):
+            elif not custom_prefetches and GenericForeignKey and isinstance(model_field, GenericForeignKey):
                 # There's not much we can do to optimize generic foreign keys regarding
                 # only/select_related because they can be anything.
                 # Just prefetch_related them
@@ -530,7 +567,8 @@ def _get_model_hints(
                 if len(f_types) > 1:
                     # This might be a generic foreign key.
                     # In this case, just prefetch it
-                    store.prefetch_related.append(model_fieldname)
+                    if not custom_prefetches:
+                        store.prefetch_related.append(model_fieldname)
                 elif len(f_types) == 1:
                     remote_field = model_field.remote_field
                     remote_model = remote_field.model
@@ -590,24 +628,37 @@ def _get_model_hints(
 
                         cache.setdefault(remote_model, []).append((level, f_store))
 
-                        # If prefetch_custom_queryset is false, use _base_manager here
-                        # instead of _default_manager because we are getting related
-                        # objects, and not querying it directly. Else use the type's
-                        # get_queryset and model's custom QuerySet.
-                        base_qs = _get_prefetch_queryset(
-                            remote_model,
-                            field,
-                            config,
-                            info,
-                        )
-                        f_qs = f_store.apply(
-                            base_qs,
-                            info=info,
-                            config=config,
-                        )
-                        f_prefetch = Prefetch(path, queryset=f_qs)
-                        f_prefetch._optimizer_sentinel = _sentinel  # type: ignore
-                        store.prefetch_related.append(f_prefetch)
+                        if custom_prefetches:
+                            for prefetch in custom_prefetches:
+                                if prefetch.queryset is not None:
+                                    p_qs = prefetch.queryset
+                                else:
+                                    p_qs = _get_prefetch_queryset(remote_model, field, config, info)
+                                f_qs = f_store.apply(p_qs, info=info, config=config)
+                                f_prefetch = Prefetch(prefetch.prefetch_through, f_qs, prefetch.to_attr)
+                                if prefix:
+                                    f_prefetch.add_prefix(prefix)
+                                f_prefetch._optimizer_sentinel = _sentinel  # type: ignore
+                                store.prefetch_related.append(f_prefetch)
+                        else:
+                            # If prefetch_custom_queryset is false, use _base_manager here
+                            # instead of _default_manager because we are getting related
+                            # objects, and not querying it directly. Else use the type's
+                            # get_queryset and model's custom QuerySet.
+                            base_qs = _get_prefetch_queryset(
+                                remote_model,
+                                field,
+                                config,
+                                info,
+                            )
+                            f_qs = f_store.apply(
+                                base_qs,
+                                info=info,
+                                config=config,
+                            )
+                            f_prefetch = Prefetch(path, queryset=f_qs)
+                            f_prefetch._optimizer_sentinel = _sentinel  # type: ignore
+                            store.prefetch_related.append(f_prefetch)
             else:
                 store.only.append(path)
 

tests/projects/models.py

@@ -1,8 +1,9 @@
-from typing import TYPE_CHECKING, Any, Optional
+from typing import TYPE_CHECKING, Any, Optional, Annotated
 
+import strawberry
 from django.contrib.auth import get_user_model
 from django.db import models
-from django.db.models import Count, QuerySet
+from django.db.models import Count, QuerySet, Prefetch
 from django.utils.translation import gettext_lazy as _
 from django_choices_field import TextChoicesField
 
@@ -54,6 +55,22 @@ class Status(models.TextChoices):
     def is_small(self) -> bool:
         return self._milestone_count < 3  # type: ignore
 
+    @model_property(
+        prefetch_related=lambda _: Prefetch(
+            "milestones",
+            to_attr="next_milestones_prop_pf",
+            queryset=Milestone.objects.filter(due_date__isnull=False).order_by("due_date")
+        )
+    )
+    def next_milestones_property(self) -> list[Annotated['MilestoneType', strawberry.lazy('.schema')]]:
+        """
+        The milestones for the project ordered by their due date
+        """
+        if hasattr(self, 'next_milestones_prop_pf'):
+            return self.next_milestones_prop_pf
+        else:
+            return self.milestones.filter(due_date__isnull=False).order_by("due_date")
+
 
 class Milestone(models.Model):
     issues: "RelatedManager[Issue]"

tests/projects/schema.py

@@ -1,7 +1,7 @@
 import asyncio
 import datetime
 import decimal
-from typing import Iterable, List, Optional, Type, cast
+from typing import Iterable, List, Optional, Type, cast, Union
 
 import strawberry
 from django.contrib.auth import get_user_model
@@ -103,6 +103,24 @@ class ProjectType(relay.Node):
     cost: strawberry.auto = strawberry_django.field(extensions=[IsAuthenticated()])
     is_small: strawberry.auto
 
+    next_milestones_property: strawberry.auto
+
+    @strawberry_django.field(
+        prefetch_related=lambda _: Prefetch(
+            "milestones",
+            to_attr="next_milestones_pf",
+            queryset=Milestone.objects.filter(due_date__isnull=False).order_by("due_date")
+        )
+    )
+    def next_milestones(self) -> "list[MilestoneType]":
+        """
+        The milestones for the project ordered by their due date
+        """
+        if hasattr(self, 'next_milestones_pf'):
+            return self.next_milestones_pf
+        else:
+            return self.milestones.filter(due_date__isnull=False).order_by("due_date")
+
 
 @strawberry_django.filter(Milestone, lookups=True)
 class MilestoneFilter:
@@ -296,6 +314,14 @@ class ProjectConnection(ListConnectionWithTotalCount[ProjectType]):
     """Project connection documentation."""
 
 
+ProjectFeedItem = Annotated[Union[IssueType, MilestoneType], strawberry.union('ProjectFeedItem')]
+
+
+@strawberry.type
+class ProjectFeedConnection(relay.Connection[ProjectFeedItem]):
+    pass
+
+
 @strawberry.type
 class Query:
     """All available queries for this schema."""

tests/test_optimizer.py

@@ -534,6 +534,96 @@ def test_query_prefetch_with_fragments(db, gql_client: GraphQLTestClient):
         assert res.data == {"project": e}
 
 
+@pytest.mark.django_db(transaction=True)
+def test_query_prefetch_with_to_attr(db, gql_client: GraphQLTestClient):
+    query = """
+      query TestQuery {
+        projectList {
+          id
+          nextMilestones {
+            id
+            name
+            project {
+              id
+              name
+            }
+          }
+        }
+      }
+    """
+
+    expected = []
+    for p in ProjectFactory.create_batch(2):
+        p_res: dict[str, Any] = {
+            "id": to_base64("ProjectType", p.id),
+            "nextMilestones": [],
+        }
+        expected.append(p_res)
+        milestones = MilestoneFactory.create_batch(2, project=p)
+        milestones.sort(key=lambda ms: ms.due_date)
+        for m in milestones:
+            m_res: dict[str, Any] = {
+                "id": to_base64("MilestoneType", m.id),
+                "name": m.name,
+                "project": {
+                    "id": p_res["id"],
+                    "name": p.name,
+                },
+            }
+            p_res["nextMilestones"].append(m_res)
+
+    assert len(expected) == 2
+
+    with assert_num_queries(2 if DjangoOptimizerExtension.enabled.get() else 3):
+        res = gql_client.query(query)
+    assert res.data == {"projectList": expected}
+
+
+@pytest.mark.django_db(transaction=True)
+def test_query_prefetch_with_to_attr_model_property(db, gql_client: GraphQLTestClient):
+    query = """
+      query TestQuery {
+        projectList {
+          id
+          nextMilestonesProperty {
+            id
+            name
+            project {
+              id
+              name
+            }
+          }
+        }
+      }
+    """
+
+    expected = []
+    for p in ProjectFactory.create_batch(2):
+        p_res: dict[str, Any] = {
+            "id": to_base64("ProjectType", p.id),
+            "nextMilestonesProperty": [],
+        }
+        expected.append(p_res)
+        milestones = MilestoneFactory.create_batch(2, project=p)
+        milestones.sort(key=lambda ms: ms.due_date)
+        for m in milestones:
+            m_res: dict[str, Any] = {
+                "id": to_base64("MilestoneType", m.id),
+                "name": m.name,
+                "project": {
+                    "id": p_res["id"],
+                    "name": p.name,
+                },
+            }
+            p_res["nextMilestonesProperty"].append(m_res)
+
+    assert len(expected) == 2
+
+    with assert_num_queries(2 if DjangoOptimizerExtension.enabled.get() else 3):
+        res = gql_client.query(query)
+    assert res.data == {"projectList": expected}
+
+
 @pytest.mark.django_db(transaction=True)
 def test_query_connection_with_resolver(db, gql_client: GraphQLTestClient):
     query = """