Fix missing as_expr

Author: WaVEV

django_mongodb_backend/aggregates.py

@@ -27,7 +27,7 @@ def aggregate(self, compiler, connection, operator=None, resolve_inner_expressio
         except NotSupportedError:
             source_expressions = self.get_source_expressions()
             condition = Case(When(self.filter.condition, then=source_expressions[0]))
-            lhs_mql = condition.as_mql(compiler, connection)
+            lhs_mql = condition.as_mql(compiler, connection, as_expr=True)
         except FullResultSet:
             lhs_mql = source_expressions[0].as_mql(compiler, connection, as_expr=True)
         except EmptyResultSet: