Fixed #21391 -- Allow model signals to lazily reference their senders.

docs/ref/signals.txt

@@ -22,7 +22,7 @@ Model signals
    :synopsis: Signals sent by the model system.
 
 The :mod:`django.db.models.signals` module defines a set of signals sent by the
-module system.
+model system.
 
 .. warning::
 
@@ -37,6 +37,14 @@ module system.
     so if your handler is a local function, it may be garbage collected.  To
     prevent this, pass ``weak=False`` when you call the signal's :meth:`~django.dispatch.Signal.connect`.
 
+.. versionadded:: 1.7
+
+    Model signals ``sender`` model can be lazily referenced when connecting a
+    receiver by specifying its full application label. For example, an
+    ``Answer`` model defined in the ``polls`` application could be referenced
+    as ``'polls.Answer'``. This sort of reference can be quite handy when
+    dealing with circular import dependencies and swappable models.
+
 pre_init
 --------
 

docs/releases/1.7.txt

@@ -425,7 +425,7 @@ Models
 * Is it now possible to avoid creating a backward relation for
   :class:`~django.db.models.OneToOneField` by setting its
   :attr:`~django.db.models.ForeignKey.related_name` to
-  `'+'` or ending it with `'+'`.
+  ``'+'`` or ending it with ``'+'``.
 
 * :class:`F expressions <django.db.models.F>` support the power operator
   (``**``).
@@ -436,6 +436,10 @@ Signals
 * The ``enter`` argument was added to the
   :data:`~django.test.signals.setting_changed` signal.
 
+* The model signals can be now be connected to using a ``str`` of the
+  ``'app_label.ModelName'`` form – just like related fields – to lazily
+  reference their senders.
+
 Templates
 ^^^^^^^^^
 

docs/topics/auth/customizing.txt

@@ -413,6 +413,19 @@ different User model.
         class Article(models.Model):
             author = models.ForeignKey(settings.AUTH_USER_MODEL)
 
+    .. versionadded:: 1.7
+
+        When connecting to signals sent by the User model, you should specify the
+        custom model using the :setting:`AUTH_USER_MODEL` setting. For example::
+
+            from django.conf import settings
+            from django.db.models.signals import post_save
+
+            def post_save_receiver(signal, sender, instance, **kwargs):
+                pass
+
+            post_save.connect(post_save_receiver, sender=settings.AUTH_USER_MODEL)
+
 Specifying a custom User model
 ------------------------------