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

Keeps precision instead of forcing DecimalField to FloatField.

django/db/models/aggregates.py

@@ -3,7 +3,8 @@ Classes to represent the definitions of aggregate functions.
 """
 from django.core.exceptions import FieldError
 from django.db.models.expressions import Case, Func, Star, When
-from django.db.models.fields import DecimalField, FloatField, IntegerField
+from django.db.models.fields import FloatField, IntegerField
+from django.db.models.functions.mixins import NumericOutputFieldMixin
 
 __all__ = [
     'Aggregate', 'Avg', 'Count', 'Max', 'Min', 'StdDev', 'Sum', 'Variance',
@@ -93,16 +94,10 @@ class Aggregate(Func):
         return options
 
 
-class Avg(Aggregate):
+class Avg(NumericOutputFieldMixin, Aggregate):
     function = 'AVG'
     name = 'Avg'
 
-    def _resolve_output_field(self):
-        source_field = self.get_source_fields()[0]
-        if isinstance(source_field, (IntegerField, DecimalField)):
-            return FloatField()
-        return super()._resolve_output_field()
-
     def as_mysql(self, compiler, connection, **extra_context):
         sql, params = super().as_sql(compiler, connection, **extra_context)
         if self.output_field.get_internal_type() == 'DurationField':

django/db/models/functions/mixins.py

@@ -1,6 +1,6 @@
 import sys
 
-from django.db.models.fields import DecimalField, FloatField
+from django.db.models.fields import DecimalField, FloatField, IntegerField
 from django.db.models.functions import Cast
 
 
@@ -23,5 +23,9 @@ class FixDecimalInputMixin:
 class NumericOutputFieldMixin:
 
     def _resolve_output_field(self):
-        has_decimals = any(isinstance(s.output_field, DecimalField) for s in self.get_source_expressions())
+        source_expressions = self.get_source_expressions()
-        return DecimalField() if has_decimals else FloatField()
+        if any(isinstance(s.output_field, DecimalField) for s in source_expressions):
+            return DecimalField()
+        if any(isinstance(s.output_field, IntegerField) for s in source_expressions):
+            return FloatField()
+        return super()._resolve_output_field() if source_expressions else FloatField()

docs/ref/models/querysets.txt

@@ -3336,14 +3336,14 @@ by the aggregate.
 ``Avg``
 ~~~~~~~
 
-.. class:: Avg(expression, output_field=FloatField(), filter=None, **extra)
+.. class:: Avg(expression, output_field=None, filter=None, **extra)
 
     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`` (or the type of whatever ``output_field`` is
+    * Return type: ``float`` if input is ``int``, otherwise same as input
-      specified)
+      field, or ``output_field`` if supplied
 
 ``Count``
 ~~~~~~~~~

docs/releases/2.2.txt

@@ -493,6 +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``.
+
 .. _deprecated-features-2.2:
 
 Features deprecated in 2.2

tests/aggregation/tests.py

@@ -865,15 +865,15 @@ class AggregateTestCase(TestCase):
 
     def test_avg_decimal_field(self):
         v = Book.objects.filter(rating=4).aggregate(avg_price=(Avg('price')))['avg_price']
-        self.assertIsInstance(v, float)
+        self.assertIsInstance(v, Decimal)
-        self.assertEqual(v, Approximate(47.39, places=2))
+        self.assertEqual(v, Approximate(Decimal('47.39'), places=2))
 
     def test_order_of_precedence(self):
         p1 = Book.objects.filter(rating=4).aggregate(avg_price=(Avg('price') + 2) * 3)
-        self.assertEqual(p1, {'avg_price': Approximate(148.18, places=2)})
+        self.assertEqual(p1, {'avg_price': Approximate(Decimal('148.18'), places=2)})
 
         p2 = Book.objects.filter(rating=4).aggregate(avg_price=Avg('price') + 2 * 3)
-        self.assertEqual(p2, {'avg_price': Approximate(53.39, places=2)})
+        self.assertEqual(p2, {'avg_price': Approximate(Decimal('53.39'), places=2)})
 
     def test_combine_different_types(self):
         msg = 'Expression contains mixed types. You must set output_field.'
@@ -1087,7 +1087,7 @@ class AggregateTestCase(TestCase):
                 return super().as_sql(compiler, connection, function='MAX', **extra_context)
 
         qs = Publisher.objects.annotate(
-            price_or_median=Greatest(Avg('book__rating'), Avg('book__price'))
+            price_or_median=Greatest(Avg('book__rating', output_field=DecimalField()), Avg('book__price'))
         ).filter(price_or_median__gte=F('num_awards')).order_by('num_awards')
         self.assertQuerysetEqual(
             qs, [1, 3, 7, 9], lambda v: v.num_awards)

tests/aggregation_regress/tests.py

@@ -401,7 +401,7 @@ class AggregationTests(TestCase):
                 When(pages__lt=400, then='discount_price'),
                 output_field=DecimalField()
             )))['test'],
-            22.27, places=2
+            Decimal('22.27'), places=2
         )
 
     def test_distinct_conditional_aggregate(self):
@@ -1041,7 +1041,7 @@ class AggregationTests(TestCase):
         books = Book.objects.values_list("publisher__name").annotate(
             Count("id"), Avg("price"), Avg("authors__age"), avg_pgs=Avg("pages")
         ).order_by("-publisher__name")
-        self.assertEqual(books[0], ('Sams', 1, 23.09, 45.0, 528.0))
+        self.assertEqual(books[0], ('Sams', 1, Decimal('23.09'), 45.0, 528.0))
 
     def test_annotation_disjunction(self):
         qs = Book.objects.annotate(n_authors=Count("authors")).filter(