Q expression always came from a column, so we don't need to check if it is.

Author: WaVEV

django_mongodb/indexes.py

@@ -1,7 +1,6 @@
 from django.core.exceptions import EmptyResultSet, FullResultSet
 from django.db import NotSupportedError
 from django.db.models import Index
-from django.db.models.expressions import Col
 from django.db.models.fields.related_lookups import In
 from django.db.models.lookups import BuiltinLookup
 from django.db.models.sql.query import Query
@@ -28,8 +27,6 @@ def _get_condition_mql(self, model, schema_editor):
 
 
 def builtin_lookup_idx(self, compiler, connection):
-    if not isinstance(self.lhs, Col):
-        raise NotSupportedError("Expressions as indexes are not supported.")
     lhs_mql = self.lhs.target.column
     value = process_rhs(self, compiler, connection)
     try: