Fixed #25609 -- Fixed regression in related field nested lookup error.

django/db/models/sql/query.py

@@ -1172,7 +1172,10 @@ class Query(object):
 
         if field.is_relation:
             # No support for transforms for relational fields
-            assert len(lookups) == 1
+            num_lookups = len(lookups)
+            if num_lookups > 1:
+                raise FieldError('Related Field got invalid lookup: {}'.format(lookups[0]))
+            assert num_lookups > 0  # Likely a bug in Django if this fails.
             lookup_class = field.get_lookup(lookups[0])
             if len(targets) == 1:
                 lhs = targets[0].get_col(alias, field)

tests/lookup/tests.py

@@ -493,6 +493,12 @@ class LookupTests(TestCase):
                 str(ex), "Unsupported lookup 'starts' for CharField "
                 "or join on the field not permitted.")
 
+    def test_relation_nested_lookup_error(self):
+        # An invalid nested lookup on a related field raises a useful error.
+        msg = 'Related Field got invalid lookup: editor'
+        with self.assertRaisesMessage(FieldError, msg):
+            Article.objects.filter(author__editor__name='James')
+
     def test_regex(self):
         # Create some articles with a bit more interesting headlines for testing field lookups:
         for a in Article.objects.all():