Refs #28643 -- Changed StdDev() to use NumericOutputFieldMixin.

Keeps precision instead of forcing DecimalField to FloatField.

django/db/models/aggregates.py

@@ -141,9 +141,8 @@ class Min(Aggregate):
     name = 'Min'
 
 
-class StdDev(Aggregate):
+class StdDev(NumericOutputFieldMixin, Aggregate):
     name = 'StdDev'
-    output_field = FloatField()
 
     def __init__(self, expression, sample=False, **extra):
         self.function = 'STDDEV_SAMP' if sample else 'STDDEV_POP'

docs/ref/models/querysets.txt

@@ -3387,12 +3387,13 @@ by the aggregate.
 ``StdDev``
 ~~~~~~~~~~
 
-.. class:: StdDev(expression, sample=False, filter=None, **extra)
+.. class:: StdDev(expression, output_field=None, sample=False, filter=None, **extra)
 
     Returns the standard deviation of the data in the provided expression.
 
     * Default alias: ``<field>__stddev``
-    * Return type: ``float``
+    * Return type: ``float`` if input is ``int``, otherwise same as input
+      field, or ``output_field`` if supplied
 
     Has one optional argument:
 

docs/releases/2.2.txt

@@ -493,8 +493,9 @@ Miscellaneous
 
 * :djadmin:`runserver` no longer supports `pyinotify` (replaced by Watchman).
 
-* The :class:`~django.db.models.Avg` aggregate function now returns a
-  ``Decimal`` instead of a ``float`` when the input is ``Decimal``.
+* The :class:`~django.db.models.Avg` and :class:`~django.db.models.StdDev`
+  aggregate functions now return a ``Decimal`` instead of a ``float`` when the
+  input is ``Decimal``.
 
 .. _deprecated-features-2.2:
 

tests/aggregation_regress/tests.py

@@ -1130,7 +1130,7 @@ class AggregationTests(TestCase):
 
         self.assertEqual(
             Book.objects.aggregate(StdDev('price')),
-            {'price__stddev': Approximate(24.16, 2)}
+            {'price__stddev': Approximate(Decimal('24.16'), 2)}
         )
 
         self.assertEqual(
@@ -1145,7 +1145,7 @@ class AggregationTests(TestCase):
 
         self.assertEqual(
             Book.objects.aggregate(StdDev('price', sample=True)),
-            {'price__stddev': Approximate(26.46, 1)}
+            {'price__stddev': Approximate(Decimal('26.46'), 1)}
         )
 
         self.assertEqual(