[3.0.x] Fixed #30796 -- Prevented select_related() from mutating a qu…

…eryset on chaining.

Thanks Darren Maki for the report.

Backport of 37f8f29 from master

django/db/models/sql/query.py

@@ -6,6 +6,7 @@
 databases). The abstraction barrier only works one way: this module has to know
 all about the internals of models in order to get the information it needs.
 """
+import copy
 import difflib
 import functools
 import inspect
@@ -324,6 +325,10 @@ def clone(self):
             obj._extra_select_cache = None
         else:
             obj._extra_select_cache = self._extra_select_cache.copy()
+        if self.select_related is not False:
+            # Use deepcopy because select_related stores fields in nested
+            # dicts.
+            obj.select_related = copy.deepcopy(obj.select_related)
         if 'subq_aliases' in self.__dict__:
             obj.subq_aliases = self.subq_aliases.copy()
         obj.used_aliases = self.used_aliases.copy()

tests/queries/test_query.py

@@ -106,3 +106,10 @@ def test_foreign_key_exclusive(self):
         self.assertIsInstance(b_isnull, RelatedIsNull)
         self.assertIsInstance(b_isnull.lhs, SimpleCol)
         self.assertEqual(b_isnull.lhs.target, ObjectC._meta.get_field('objectb'))
+
+    def test_clone_select_related(self):
+        query = Query(Item)
+        query.add_select_related(['creator'])
+        clone = query.clone()
+        clone.add_select_related(['note', 'creator__extra'])
+        self.assertEqual(query.select_related, {'creator': {}})