feat: Avoid joins when only fetching an ID

strawberry_django/fields/field.py

@@ -183,7 +183,24 @@
                     result = getattr(source, attr.field.attname)
                 elif isinstance(attr, ForwardManyToOneDescriptor):
                     # This will raise KeyError if it is not cached
-                    result = attr.field.get_cached_value(source)  # type: ignore
+                    try:
+                        result = attr.field.get_cached_value(source)  # type: ignore
+                    except KeyError:
+                        selected_field = next(
+                            f for f in info.selected_fields if f.name == info.field_name
+                        )
+                        target_attname = attr.field.target_field.attname
+                        if (
+                            len(selected_field.selections) == 1
+                            and selected_field.selections[0].name == target_attname
+                        ):
+                            # If we are only retrieving the referenced `id` of a related foreignkey with no additional columns
+                            # then we can optimize away this retrieval by reusing the existing value from the `source` object.
+                            value = source.__dict__[attr.field.get_attname()]
+                            target_model = attr.field.target_field.model
+                            result = target_model(pk=value)
+                        else:
+                            raise
                 elif isinstance(attr, ReverseOneToOneDescriptor):
                     # This will raise KeyError if it is not cached
                     result = attr.related.get_cached_value(source)

strawberry_django/optimizer.py

@@ -492,7 +492,6 @@ def _get_model_hints(
 
             if isinstance(model_field, (models.ForeignKey, OneToOneRel)):
                 store.only.append(path)
-                store.select_related.append(path)
 
                 # If adding a reverse relation, make sure to select its pointer to us,
                 # or else this might causa a refetch from the database
@@ -514,7 +513,10 @@ def _get_model_hints(
                         cache=cache,
                         level=level + 1,
                     )
-                    if f_store is not None:
+                    if f_store is not None and set(f_store.only) != {
+                        model_field.target_field.attname
+                    }:
+                        store.select_related.append(path)
                         cache.setdefault(f_model, []).append((level, f_store))
                         store |= f_store.with_prefix(path, info=info)
             elif GenericForeignKey and isinstance(model_field, GenericForeignKey):