Skip test.

Author: WaVEV

django_mongodb/features.py

@@ -385,6 +385,7 @@ def django_test_expected_failures(self):
             "aggregation.tests.AggregateTestCase.test_aggregation_subquery_annotation",
             "aggregation.tests.AggregateTestCase.test_aggregation_subquery_annotation_related_field",
             "delete.tests.DeletionTests.test_only_referenced_fields_selected",
+            "expressions.tests.ExistsTests.test_optimizations",
             "lookup.tests.LookupTests.test_in_ignore_none",
             "lookup.tests.LookupTests.test_textfield_exact_null",
             "migrations.test_commands.MigrateTests.test_migrate_syncdb_app_label",