framework/rbac/migrations/m170907_052038_rbac_add_index_on_auth_assignment_user_id.php
public function down()
    {
        $authManager = $this->getAuthManager();
        $this->db = $authManager->db;

        $this->dropIndex($this->index, $authManager->assignmentTable);
    }
public function down()
    {
        $authManager = $this->getAuthManager();
        $this->db = $authManager->db;

        $this->dropIndex($this->index, $authManager->assignmentTable);
    }
{@inheritdoc}