Fixed #24485 -- Allowed combined expressions to set output_field

2 files changed, 29 insertions, 4 deletions

diff --git a/django/db/models/__init__.py b/django/db/models/__init__.py
index fea271f403..4d8ea38898 100644
--- a/django/db/models/__init__.py
+++ b/django/db/models/__init__.py
@@ -2,7 +2,9 @@ from functools import wraps
 
 from django.core.exceptions import ObjectDoesNotExist, ImproperlyConfigured  # NOQA
 from django.db.models.query import Q, QuerySet, Prefetch  # NOQA
-from django.db.models.expressions import Expression, F, Value, Func, Case, When  # NOQA
+from django.db.models.expressions import (  # NOQA
+    Expression, ExpressionWrapper, F, Value, Func, Case, When,
+)
 from django.db.models.manager import Manager  # NOQA
 from django.db.models.base import Model  # NOQA
 from django.db.models.aggregates import *  # NOQA
diff --git a/django/db/models/expressions.py b/django/db/models/expressions.py
index c48204f420..c6959f87b8 100644
--- a/django/db/models/expressions.py
+++ b/django/db/models/expressions.py
@@ -126,12 +126,12 @@ class BaseExpression(object):
     # aggregate specific fields
     is_summary = False
 
-    def get_db_converters(self, connection):
-        return [self.convert_value] + self.output_field.get_db_converters(connection)
-
     def __init__(self, output_field=None):
         self._output_field = output_field
 
+    def get_db_converters(self, connection):
+        return [self.convert_value] + self.output_field.get_db_converters(connection)
+
     def get_source_expressions(self):
         return []
 
@@ -656,6 +656,29 @@ class Ref(Expression):
         return [self]
 
 
+class ExpressionWrapper(Expression):
+    """
+    An expression that can wrap another expression so that it can provide
+    extra context to the inner expression, such as the output_field.
+    """
+
+    def __init__(self, expression, output_field):
+        super(ExpressionWrapper, self).__init__(output_field=output_field)
+        self.expression = expression
+
+    def set_source_expressions(self, exprs):
+        self.expression = exprs[0]
+
+    def get_source_expressions(self):
+        return [self.expression]
+
+    def as_sql(self, compiler, connection):
+        return self.expression.as_sql(compiler, connection)
+
+    def __repr__(self):
+        return "{}({})".format(self.__class__.__name__, self.expression)
+
+
 class When(Expression):
     template = 'WHEN %(condition)s THEN %(result)s'