Ticket #15494: only_pk_related.diff

django/db/models/query.py

@@ -708 +708 @@
             # Can only pass None to defer(), not only(), as the rest option.
             # That won't stop people trying to do this, so let's be explicit.
             raise TypeError("Cannot pass None as an argument to only().")
+
+        cleaned_fields = []
+        for field_name in fields:
+            if field_name == 'pk':
+                field_name = self.model._meta.pk.name
+            cleaned_fields.append(field_name)
+
         clone = self._clone()
-        clone.query.add_immediate_loading(fields)
+        clone.query.add_immediate_loading(cleaned_fields)
         return clone
 
     def using(self, alias):