Fixed #36683 -- Added error message on QuerySet.update() following distinct(*fields).

AUTHORS

@@ -707,6 +707,7 @@ answer newbie questions, and generally made Django that much better:
     Matt Dennenbaum
     Matthew Flanagan <https://wadofstuff.blogspot.com/>
     Matthew Schinckel <matt@schinckel.net>
+    Matthew Shirley <matt@mattshirley.net>
     Matthew Somerville <matthew-django@dracos.co.uk>
     Matthew Tretter <m@tthewwithanm.com>
     Matthew Wilkes <matt@matthewwilkes.name>

django/db/models/query.py

@@ -1338,6 +1338,8 @@ class QuerySet(AltersData):
         self._not_support_combined_queries("update")
         if self.query.is_sliced:
             raise TypeError("Cannot update a query once a slice has been taken.")
+        if self.query.distinct_fields:
+            raise TypeError("Cannot call update() after .distinct(*fields).")
         self._for_write = True
         query = self.query.chain(sql.UpdateQuery)
         query.add_update_values(kwargs)

tests/distinct_on_fields/tests.py

@@ -178,3 +178,9 @@ class DistinctOnTests(TestCase):
             .order_by("nAmEAlIaS")
         )
         self.assertSequenceEqual(qs, [self.p1_o1, self.p2_o1, self.p3_o1])
+
+    def test_disallowed_update_distinct_on(self):
+        qs = Staff.objects.distinct("organisation").order_by("organisation")
+        msg = "Cannot call update() after .distinct(*fields)."
+        with self.assertRaisesMessage(TypeError, msg):
+            qs.update(name="p4")