Modified readiness check in AppConfig.get_model(s).

It was inconsistent with the equivalent check in Apps.get_model(s)
because I made incorrect assumptions when I wrote that code and
needlessly complicated readiness checks.

This is a backwards-incompatible change.

docs/ref/applications.txt

@@ -227,11 +227,16 @@ Methods
     Returns an iterable of :class:`~django.db.models.Model` classes for this
     application.
 
+    Requires the app registry to be fully populated.
+
 .. method:: AppConfig.get_model(model_name)
 
     Returns the :class:`~django.db.models.Model` with the given
-    ``model_name``. Raises :exc:`LookupError` if no such model exists in this
-    application. ``model_name`` is case-insensitive.
+    ``model_name``. ``model_name`` is case-insensitive.
+
+    Raises :exc:`LookupError` if no such model exists in this application.
+
+    Requires the app registry to be fully populated.
 
 .. method:: AppConfig.ready()
 

docs/releases/1.11.txt

@@ -579,6 +579,13 @@ Miscellaneous
 * ``ConditionalGetMiddleware`` no longer sets the ``Date`` header as Web
   servers set that header.
 
+* :meth:`~django.apps.AppConfig.get_model` and
+  :meth:`~django.apps.AppConfig.get_models` now raise
+  :exc:`~django.core.exceptions.AppRegistryNotReady` if they're called before
+  models of all applications have been loaded. Previously they only required
+  the target application's models to be loaded and thus could return models
+  without all their relations set up.
+
 .. _deprecated-features-1.11:
 
 Features deprecated in 1.11