Fixed #24649 -- Allowed using Avg aggregate on non-numeric field types.

docs/ref/models/querysets.txt

@@ -2802,12 +2802,19 @@ by the aggregate.
 Avg
 ~~~
 
-.. class:: Avg(expression, output_field=None, **extra)
+.. class:: Avg(expression, output_field=FloatField(), **extra)
 
-    Returns the mean value of the given expression, which must be numeric.
+    Returns the mean value of the given expression, which must be numeric
+    unless you specify a different ``output_field``.
 
     * Default alias: ``<field>__avg``
-    * Return type: ``float``
+    * Return type: ``float`` (or the type of whatever ``output_field`` is
+      specified)
+
+    .. versionchanged:: 1.9
+
+        The ``output_field`` parameter was added to allow aggregating over
+        non-numeric columns, such as ``DurationField``.
 
 Count
 ~~~~~

docs/releases/1.9.txt

@@ -200,6 +200,10 @@ Models
   (such as :lookup:`exact`, :lookup:`gt`, :lookup:`lt`, etc.). For example:
   ``Entry.objects.filter(pub_date__month__gt=6)``.
 
+* You can specify the ``output_field`` parameter of the
+  :class:`~django.db.models.Avg` aggregate in order to aggregate over
+  non-numeric columns, such as ``DurationField``.
+
 CSRF
 ^^^^