DBEntry 用orm的方式保存数据出错

保存代码：UserInfo user = new UserInfo();
            user.user_id = 5;
            user.user_name = "a";
            DbEntry.Save(user);对象模型： public class UserInfo : DbObject
    {
        [DbKey(IsDbGenerate = false)]
        public int user_id;
        public string user_name;
    }
在DbEntry.Save(user)；的时候抛出 “Multiple key do not allow SystemGeneration!” 异常