[3.0.x] Fixed #30796 -- Prevented select_related() from mutating a queryset on chaining.

Thanks Darren Maki for the report.

Backport of 37f8f293775d0b672da8ae369d9a4e17f1db7851 from master

1 files changed, 5 insertions, 0 deletions

diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 8e55bd092d..ea51912e8b 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -6,6 +6,7 @@ themselves do not have to (and could be backed by things other than SQL
 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 @@ class Query(BaseExpression):
             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()