Refs #26709 -- Made Index raise ValueError on non-string fields.

django/db/models/indexes.py

@@ -35,6 +35,8 @@ class Index:
             raise ValueError('Index.fields and Index.opclasses must have the same number of elements.')
         if not fields:
             raise ValueError('At least one field is required to define an index.')
+        if not all(isinstance(field, str) for field in fields):
+            raise ValueError('Index.fields must contain only strings with field names.')
         if include and not name:
             raise ValueError('A covering index must be named.')
         if not isinstance(include, (type(None), list, tuple)):

tests/model_indexes/tests.py

@@ -55,6 +55,11 @@ class SimpleIndexesTests(SimpleTestCase):
         with self.assertRaisesMessage(ValueError, 'Index.fields must be a list or tuple.'):
             models.Index(fields='title')
 
+    def test_index_fields_strings(self):
+        msg = 'Index.fields must contain only strings with field names.'
+        with self.assertRaisesMessage(ValueError, msg):
+            models.Index(fields=[models.F('title')])
+
     def test_fields_tuple(self):
         self.assertEqual(models.Index(fields=('title',)).fields, ['title'])