From e85afa5943695457c85e9bc1c5dc0d985004e303 Mon Sep 17 00:00:00 2001
From: Nick Pope <nick.pope@flightdataservices.com>
Date: Wed, 19 Dec 2018 23:03:42 +0000
Subject: [PATCH] Refs #28643 -- Changed StdDev() to use
 NumericOutputFieldMixin.

Keeps precision instead of forcing DecimalField to FloatField.
---
 django/db/models/aggregates.py     | 3 +--
 docs/ref/models/querysets.txt      | 5 +++--
 docs/releases/2.2.txt              | 5 +++--
 tests/aggregation_regress/tests.py | 4 ++--
 4 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/django/db/models/aggregates.py b/django/db/models/aggregates.py
index f9202543a3..9e4a71ef68 100644
--- a/django/db/models/aggregates.py
+++ b/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'
diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt
index 46fcd50e37..a80528220c 100644
--- a/docs/ref/models/querysets.txt
+++ b/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:
 
diff --git a/docs/releases/2.2.txt b/docs/releases/2.2.txt
index b96b0ed1ef..7323fd6e42 100644
--- a/docs/releases/2.2.txt
+++ b/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:
 
diff --git a/tests/aggregation_regress/tests.py b/tests/aggregation_regress/tests.py
index 64bbc13f80..d72e75ce32 100644
--- a/tests/aggregation_regress/tests.py
+++ b/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(