Fixed #36373 -- Fixed select_related() crash on foreign object for a composite pk.

Thanks Jacob Walls for the report and Sarah for the in-depth review.

1 files changed, 5 insertions, 1 deletions

diff --git a/django/db/models/query.py b/django/db/models/query.py
index 4ba96928d4..663f8bade4 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -2678,7 +2678,11 @@ class RelatedPopulator:
             )
 
         self.model_cls = klass_info["model"]
-        self.pk_idx = self.init_list.index(self.model_cls._meta.pk.attname)
+        # A primary key must have all of its constituents not-NULL as
+        # NULL != NULL and thus NULL cannot be referenced through a foreign
+        # relationship. Therefore checking for a single member of the primary
+        # key is enough to determine if the referenced object exists or not.
+        self.pk_idx = self.init_list.index(self.model_cls._meta.pk_fields[0].attname)
         self.related_populators = get_related_populators(klass_info, select, self.db)
         self.local_setter = klass_info["local_setter"]
         self.remote_setter = klass_info["remote_setter"]