yii\mongodb\rbac\MongoDbManager::getAssignment PHP Method

    public function getAssignment($roleName, $userId)
    {
        if (empty($userId)) {
            return null;
        }
        $row = (new Query())->from($this->assignmentCollection)->where(['user_id' => (string) $userId, 'item_name' => $roleName])->one($this->db);
        if ($row === false) {
            return null;
        }
        return new Assignment(['userId' => $row['user_id'], 'roleName' => $row['item_name'], 'createdAt' => $row['created_at']]);
    }