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

django/db/backends/base/features.py

@@ -160,6 +160,9 @@ class BaseDatabaseFeatures(object):
     # Support for the DISTINCT ON clause
     can_distinct_on_fields = False
 
+    # Can the backend use an Avg aggregate on DurationField?
+    can_avg_on_durationfield = True
+
     # Does the backend decide to commit before SAVEPOINT statements
     # when autocommit is disabled? http://bugs.python.org/issue8145#msg109965
     autocommits_when_autocommit_is_off = False

django/db/backends/oracle/features.py

@@ -40,6 +40,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     uppercases_column_names = True
     # select for update with limit can be achieved on Oracle, but not with the current backend.
     supports_select_for_update_with_limit = False
+    can_avg_on_durationfield = False  # Pending implementation (#24699).
 
     def introspected_boolean_field_type(self, field=None, created_separately=False):
         """

django/db/models/aggregates.py

@@ -75,12 +75,8 @@ class Avg(Aggregate):
     name = 'Avg'
 
     def __init__(self, expression, **extra):
-        super(Avg, self).__init__(expression, output_field=FloatField(), **extra)
-
-    def convert_value(self, value, expression, connection, context):
-        if value is None:
-            return value
-        return float(value)
+        output_field = extra.pop('output_field', FloatField())
+        super(Avg, self).__init__(expression, output_field=output_field, **extra)
 
 
 class Count(Aggregate):