Fixed #25252 -- Added friendly error message on incorrect .select_related() use

Using select_related() after .values() or .values_list() is not possible
because .values() or .values_list() already got the values they need.

django/db/models/query.py

@@ -843,6 +843,10 @@ class QuerySet(object):
 
         If select_related(None) is called, the list is cleared.
         """
+
+        if self._fields is not None:
+            raise TypeError("Cannot call select_related() after .values() or .values_list()")
+
         obj = self._clone()
         if fields == (None,):
             obj.query.select_related = False

tests/select_related/tests.py

@@ -163,6 +163,26 @@ class SelectRelatedTests(TestCase):
             self.assertEqual(obj.parent_1, parent_1)
             self.assertEqual(obj.parent_2, parent_2)
 
+    def test_select_related_after_values(self):
+        """
+        Running select_related() after calling values() raises a TypeError
+        """
+
+        message = "Cannot call select_related() after .values() or .values_list()"
+
+        with self.assertRaisesMessage(TypeError, message):
+            list(Species.objects.values('name').select_related('genus'))
+
+    def test_select_related_after_values_list(self):
+        """
+        Running select_related() after calling values_list() raises a TypeError
+        """
+
+        message = "Cannot call select_related() after .values() or .values_list()"
+
+        with self.assertRaisesMessage(TypeError, message):
+            list(Species.objects.values_list('name').select_related('genus'))
+
 
 class SelectRelatedValidationTests(SimpleTestCase):
     """