Refs #20203 -- Allowed adding custom default manager to the model state

If the only manager on the model is the default manager defined
by Django (`objects = models.Manager()`), this manager will not
be added to the model state. If it is custom, it needs to be
passed to the model state.

django/db/migrations/state.py

@@ -465,7 +465,10 @@ class ModelState(object):
                 (name, instance) for name, (cc, instance) in
                 sorted(managers_mapping.items(), key=lambda v: v[1])
             ]
-            if managers == [(default_manager_name, models.Manager())]:
+            # If the only manager on the model is the default manager defined
+            # by Django (`objects = models.Manager()`), this manager will not
+            # be added to the model state.
+            if managers == [('objects', models.Manager())]:
                 managers = []
         else:
             managers = []

tests/migrations/test_state.py

@@ -166,6 +166,26 @@ class StateTests(SimpleTestCase):
         self.assertEqual([mgr.args for name, mgr in food_order_manager_state.managers],
                          [('a', 'b', 1, 2), ('x', 'y', 3, 4)])
 
+    def test_custom_default_manager_added_to_the_model_state(self):
+        """
+        When the default manager of the model is a custom manager,
+        it needs to be added to the model state.
+        """
+        new_apps = Apps(['migrations'])
+        custom_manager = models.Manager()
+
+        class Author(models.Model):
+            objects = models.TextField()
+            authors = custom_manager
+
+            class Meta:
+                app_label = 'migrations'
+                apps = new_apps
+
+        project_state = ProjectState.from_apps(new_apps)
+        author_state = project_state.models['migrations', 'author']
+        self.assertEqual(author_state.managers, [('authors', custom_manager)])
+
     def test_apps_bulk_update(self):
         """
         StateApps.bulk_update() should update apps.ready to False and reset