Fixed #28051 -- Made migrations respect Index's name argument.

Thanks Marc Tamlyn for the report and Tim Graham for the review.

django/db/migrations/state.py

@@ -446,7 +446,8 @@ class ModelState:
                 elif name == "indexes":
                     indexes = [idx.clone() for idx in model._meta.indexes]
                     for index in indexes:
-                        index.set_name_with_model(model)
+                        if not index.name:
+                            index.set_name_with_model(model)
                     options['indexes'] = indexes
                 else:
                     options[name] = model._meta.original_attrs[name]

docs/releases/1.11.1.txt

@@ -9,4 +9,6 @@ Django 1.11.1 fixes several bugs in 1.11.
 Bugfixes
 ========
 
-* ...
+* Made migrations respect ``Index``’s ``name`` argument. If you created a
+  named index with Django 1.11, ``makemigrations`` will create a migration to
+  recreate the index with the correct name (:ticket:`28051`).

tests/migrations/test_state.py

@@ -1070,6 +1070,19 @@ class ModelStateTests(SimpleTestCase):
         child1_state.options['indexes'][0].name = 'bar'
         self.assertEqual(Child1._meta.indexes[0].name, 'migrations__name_b0afd7_idx')
 
+    @isolate_apps('migrations')
+    def test_explicit_index_name(self):
+        class TestModel(models.Model):
+            name = models.CharField(max_length=50)
+
+            class Meta:
+                app_label = 'migrations'
+                indexes = [models.indexes.Index(fields=['name'], name='foo_idx')]
+
+        model_state = ModelState.from_model(TestModel)
+        index_names = [index.name for index in model_state.options['indexes']]
+        self.assertEqual(index_names, ['foo_idx'])
+
 
 class RelatedModelsTests(SimpleTestCase):