Fixed #24141 -- Added QuerySet.contains().

django/db/models/query.py

@@ -808,6 +808,27 @@ class QuerySet:
             return self.query.has_results(using=self.db)
         return bool(self._result_cache)
 
+    def contains(self, obj):
+        """Return True if the queryset contains an object."""
+        self._not_support_combined_queries('contains')
+        if self._fields is not None:
+            raise TypeError(
+                'Cannot call QuerySet.contains() after .values() or '
+                '.values_list().'
+            )
+        try:
+            if obj._meta.concrete_model != self.model._meta.concrete_model:
+                return False
+        except AttributeError:
+            raise TypeError("'obj' must be a model instance.")
+        if obj.pk is None:
+            raise ValueError(
+                'QuerySet.contains() cannot be used on unsaved objects.'
+            )
+        if self._result_cache is not None:
+            return obj in self._result_cache
+        return self.filter(pk=obj.pk).exists()
+
     def _prefetch_related_objects(self):
         # This method can only be called once the result cache has been filled.
         prefetch_related_objects(self._result_cache, *self._prefetch_related_lookups)