Fixed #30548 -- Improved exception when expression contains mixed types.

Author: CruxBox

django/db/models/expressions.py

@@ -286,8 +286,15 @@ def _resolve_output_field(self):
         """
         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.')
+            for source in sources_iter:
+                if not isinstance(output_field, source.__class__):
+                    raise FieldError(
+                        'Expression contains mixed types: %s, %s. You must '
+                        'set output_field.' % (
+                            output_field.__class__.__name__,
+                            source.__class__.__name__,
+                        )
+                    )
             return output_field
 
     @staticmethod

tests/aggregation/tests.py

@@ -884,7 +884,10 @@ def test_order_of_precedence(self):
         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.'
+        msg = (
+            'Expression contains mixed types: FloatField, IntegerField. '
+            'You must set output_field.'
+        )
         qs = Book.objects.annotate(sums=Sum('rating') + Sum('pages') + Sum('price'))
         with self.assertRaisesMessage(FieldError, msg):
             qs.first()