Fixed #21283 -- Added support for migrations if models is a package.

Thanks Markus Holtermann for the report.

tests/migrations/test_writer.py

@@ -2,12 +2,15 @@
 
 from __future__ import unicode_literals
 
+import copy
 import datetime
+import os
 
-from django.utils import six
-from django.test import TestCase
-from django.db.migrations.writer import MigrationWriter
 from django.db import models, migrations
+from django.db.migrations.writer import MigrationWriter
+from django.db.models.loading import cache
+from django.test import TestCase, override_settings
+from django.utils import six
 from django.utils.translation import ugettext_lazy as _
 
 
@@ -95,3 +98,24 @@ class WriterTests(TestCase):
         # Just make sure it runs for now, and that things look alright.
         result = self.safe_exec(output)
         self.assertIn("Migration", result)
+
+    def test_migration_path(self):
+        _old_app_store = copy.deepcopy(cache.app_store)
+
+        test_apps = [
+            'migrations.migrations_test_apps.normal',
+            'migrations.migrations_test_apps.with_package_model',
+        ]
+
+        base_dir = os.path.dirname(os.path.dirname(__file__))
+
+        try:
+            with override_settings(INSTALLED_APPS=test_apps):
+                for app in test_apps:
+                    cache.load_app(app)
+                    migration = migrations.Migration('0001_initial', app.split('.')[-1])
+                    expected_path = os.path.join(base_dir, *(app.split('.') + ['migrations', '0001_initial.py']))
+                    writer = MigrationWriter(migration)
+                    self.assertEqual(writer.path, expected_path)
+        finally:
+            cache.app_store = _old_app_store