Fixed #28382 -- Prevented BaseExpression._output_field from being set if _resolve_output_field() fails.

django/db/models/aggregates.py

@@ -44,8 +44,8 @@ class Avg(Aggregate):
     def _resolve_output_field(self):
         source_field = self.get_source_fields()[0]
         if isinstance(source_field, (IntegerField, DecimalField)):
-            self._output_field = FloatField()
-        super()._resolve_output_field()
+            return FloatField()
+        return super()._resolve_output_field()
 
     def as_oracle(self, compiler, connection):
         if self.output_field.get_internal_type() == 'DurationField':

django/db/models/expressions.py

@@ -236,7 +236,7 @@ class BaseExpression:
         None values.
         """
         if self._output_field is None:
-            self._resolve_output_field()
+            self._output_field = self._resolve_output_field()
         return self._output_field
 
     def _resolve_output_field(self):
@@ -253,18 +253,11 @@ class BaseExpression:
         this check. If all sources are `None`, then an error will be thrown
         higher up the stack in the `output_field` property.
         """
-        if self._output_field is None:
-            sources = self.get_source_fields()
-            num_sources = len(sources)
-            if num_sources == 0:
-                self._output_field = None
-            else:
-                for source in sources:
-                    if self._output_field is None:
-                        self._output_field = source
-                    if source is not None and not isinstance(self._output_field, source.__class__):
-                        raise FieldError(
-                            "Expression contains mixed types. You must set output_field")
+        sources_iter = (source for source in self.get_source_fields() if source is not None)
+        for output_field in sources_iter:
+            if any(not isinstance(output_field, source.__class__) for source in sources_iter):
+                raise FieldError('Expression contains mixed types. You must set output_field.')
+            return output_field
 
     def convert_value(self, value, expression, connection, context):
         """

tests/aggregation/tests.py

@@ -965,8 +965,12 @@ class AggregateTestCase(TestCase):
         self.assertEqual(p2, {'avg_price': Approximate(53.39, places=2)})
 
     def test_combine_different_types(self):
-        with self.assertRaisesMessage(FieldError, 'Expression contains mixed types. You must set output_field'):
-            Book.objects.annotate(sums=Sum('rating') + Sum('pages') + Sum('price')).get(pk=self.b4.pk)
+        msg = 'Expression contains mixed types. You must set output_field.'
+        qs = Book.objects.annotate(sums=Sum('rating') + Sum('pages') + Sum('price'))
+        with self.assertRaisesMessage(FieldError, msg):
+            qs.first()
+        with self.assertRaisesMessage(FieldError, msg):
+            qs.first()
 
         b1 = Book.objects.annotate(sums=Sum(F('rating') + F('pages') + F('price'),
                                    output_field=IntegerField())).get(pk=self.b4.pk)