Fix refs and get_columns as a set.

Author: WaVEV

django_mongodb/compiler.py

@@ -185,7 +185,7 @@ def _get_group_expressions(self, order_by):
             for expr, (_, _, is_ref) in order_by:
                 # Skip references.
                 if not is_ref:
-                    expressions.extend(expr.get_group_by_cols())
+                    expressions |= set(expr.get_group_by_cols())
         having_group_by = self.having.get_group_by_cols() if self.having else ()
         for expr in having_group_by:
             expressions.add(expr)

django_mongodb/expressions.py

@@ -133,9 +133,7 @@ def raw_sql(self, compiler, connection):  # noqa: ARG001
     raise NotSupportedError("RawSQL is not supported on MongoDB.")
 
 
-def ref(self, compiler, connection):
-    if isinstance(self.source, Subquery):
-        return self.source.as_mql(compiler, connection)
+def ref(self, compiler, connection):  # noqa: ARG001
     prefix = (
         f"{self.source.alias}."
         if isinstance(self.source, Col) and self.source.alias != compiler.collection_name