Handle ref expression in ordering.

Author: WaVEV

django_mongodb/compiler.py

@@ -452,12 +452,15 @@ def preprocess_orderby(self):
         result = SON()
         idx = itertools.count(start=1)
         for order in self._order_by or []:
-            if isinstance(order.expression, Col | Ref):
+            if isinstance(order.expression, Col):
                 fieldname = order.expression.as_mql(self, self.connection).removeprefix("$")
                 fields[fieldname] = order.expression
+            elif isinstance(order.expression, Ref):
+                fieldname = order.expression.as_mql(self, self.connection).removeprefix("$")
+                orderby_annotations[fieldname] = order.expression
             else:
                 fieldname = f"__order{next(idx)}"
-                orderby_annotations[fieldname] = order.expression
+                fields[fieldname] = order.expression
 
             if order.nulls_first or order.nulls_last:
                 null_fieldname = f"__order{next(idx)}"