Refs #29908 -- Optimized known related objects assignment.

Since CPython implements a C level attrgetter(*attrs) it even outperforms the
most common case of a single known related object since the resulting attribute
values tuple is built in C.

django/db/models/fields/related_descriptors.py

@@ -63,6 +63,7 @@ and two directions (forward and reverse) for a total of six combinations.
    ``ReverseManyToManyDescriptor``, use ``ManyToManyDescriptor`` instead.
 """
 
+from django.core.exceptions import FieldError
 from django.db import connections, router, transaction
 from django.db.models import Q, signals
 from django.db.models.query import QuerySet
@@ -581,10 +582,17 @@ def create_reverse_many_to_one_manager(superclass, rel):
                 # that abuse create_reverse_many_to_one_manager() with reverse
                 # one-to-many relationships instead and break known related
                 # objects assignment.
-                rel_obj_id = tuple([
-                    getattr(self.instance, target_field.attname)
-                    for target_field in self.field.get_path_info()[-1].target_fields
-                ])
+                try:
+                    target_field = self.field.target_field
+                except FieldError:
+                    # The relationship has multiple target fields. Use a tuple
+                    # for related object id.
+                    rel_obj_id = tuple([
+                        getattr(self.instance, target_field.attname)
+                        for target_field in self.field.get_path_info()[-1].target_fields
+                    ])
+                else:
+                    rel_obj_id = getattr(self.instance, target_field.attname)
                 queryset._known_related_objects = {self.field: {rel_obj_id: self.instance}}
             return queryset
 

django/db/models/query.py

@@ -62,14 +62,12 @@ class ModelIterable(BaseIterable):
                      for f in select[model_fields_start:model_fields_end]]
         related_populators = get_related_populators(klass_info, select, db)
         known_related_objects = [
-            (field, related_objs, [
-                operator.attrgetter(
-                    field.attname
-                    if from_field == 'self' else
-                    queryset.model._meta.get_field(from_field).attname
-                )
+            (field, related_objs, operator.attrgetter(*[
+                field.attname
+                if from_field == 'self' else
+                queryset.model._meta.get_field(from_field).attname
                 for from_field in field.from_fields
-            ]) for field, related_objs in queryset._known_related_objects.items()
+            ])) for field, related_objs in queryset._known_related_objects.items()
         ]
         for row in compiler.results_iter(results):
             obj = model_cls.from_db(db, init_list, row[model_fields_start:model_fields_end])
@@ -80,11 +78,11 @@ class ModelIterable(BaseIterable):
                     setattr(obj, attr_name, row[col_pos])
 
             # Add the known related objects to the model.
-            for field, rel_objs, rel_getters in known_related_objects:
+            for field, rel_objs, rel_getter in known_related_objects:
                 # Avoid overwriting objects loaded by, e.g., select_related().
                 if field.is_cached(obj):
                     continue
-                rel_obj_id = tuple([rel_getter(obj) for rel_getter in rel_getters])
+                rel_obj_id = rel_getter(obj)
                 try:
                     rel_obj = rel_objs[rel_obj_id]
                 except KeyError: