Ticket #24141: queryset-contains.diff

django/db/models/query.py

@@ -577 +577 @@
         if self._result_cache is None:
             return self.query.has_results(using=self.db)
         return bool(self._result_cache)
+    
+    def contains(self, obj):
+        if self._result_cache is not None:
+            return obj in self._result_cache
+        return isinstance(obj, self.model) and self.filter(pk=obj.pk).exists()
 
     def _prefetch_related_objects(self):
         # This method can only be called once the result cache has been filled.