Made it possible to change an application's label in its configuration.

Fixed #21683.

django/apps/registry.py

@@ -201,6 +201,27 @@ class Apps(object):
         app_config = self.app_configs.get(app_name.rpartition(".")[2])
         return app_config is not None and app_config.name == app_name
 
+    def get_containing_app_config(self, object_name):
+        """
+        Look for an app config containing a given object.
+
+        object_name is the dotted Python path to the object.
+
+        Returns the app config for the inner application in case of nesting.
+        Returns None if the object isn't in any registered app config.
+
+        It's safe to call this method at import time, even while the registry
+        is being populated.
+        """
+        candidates = []
+        for app_config in self.app_configs.values():
+            if object_name.startswith(app_config.name):
+                subpath = object_name[len(app_config.name):]
+                if subpath == '' or subpath[0] == '.':
+                    candidates.append(app_config)
+        if candidates:
+            return sorted(candidates, key=lambda ac: -len(ac.name))[0]
+
     def get_registered_model(self, app_label, model_name):
         """
         Similar to get_model(), but doesn't require that an app exists with

django/db/migrations/state.py

@@ -76,9 +76,7 @@ class AppConfigStub(AppConfig):
     Stubs a Django AppConfig. Only provides a label and a dict of models.
     """
     def __init__(self, label):
-        self.label = label
-        self.path = None
-        super(AppConfigStub, self).__init__(None, None)
+        super(AppConfigStub, self).__init__(label, None)
 
     def import_models(self, all_models):
         self.models = all_models

django/db/models/base.py

@@ -86,23 +86,35 @@ class ModelBase(type):
             meta = attr_meta
         base_meta = getattr(new_class, '_meta', None)
 
+        # Look for an application configuration to attach the model to.
+        app_config = apps.get_containing_app_config(module)
+
         if getattr(meta, 'app_label', None) is None:
-            # Figure out the app_label by looking one level up from the package
-            # or module named 'models'. If no such package or module exists,
-            # fall back to looking one level up from the module this model is
-            # defined in.
 
-            # For 'django.contrib.sites.models', this would be 'sites'.
-            # For 'geo.models.places' this would be 'geo'.
+            if app_config is None:
+                # If the model is imported before the configuration for its
+                # application is created (#21719), or isn't in an installed
+                # application (#21680), use the legacy logic to figure out the
+                # app_label by looking one level up from the package or module
+                # named 'models'. If no such package or module exists, fall
+                # back to looking one level up from the module this model is
+                # defined in.
+
+                # For 'django.contrib.sites.models', this would be 'sites'.
+                # For 'geo.models.places' this would be 'geo'.
+
+                model_module = sys.modules[new_class.__module__]
+                package_components = model_module.__name__.split('.')
+                package_components.reverse()  # find the last occurrence of 'models'
+                try:
+                    app_label_index = package_components.index(MODELS_MODULE_NAME) + 1
+                except ValueError:
+                    app_label_index = 1
+                kwargs = {"app_label": package_components[app_label_index]}
+
+            else:
+                kwargs = {"app_label": app_config.label}
 
-            model_module = sys.modules[new_class.__module__]
-            package_components = model_module.__name__.split('.')
-            package_components.reverse()  # find the last occurrence of 'models'
-            try:
-                app_label_index = package_components.index(MODELS_MODULE_NAME) + 1
-            except ValueError:
-                app_label_index = 1
-            kwargs = {"app_label": package_components[app_label_index]}
         else:
             kwargs = {}