User.findOne({userid:account.user},function(err,docs){
docs.accounts=account;
User.findOne({}).populate('accounts').exec(function(err, doc){
console.log('after populate: ', err, doc);
});
});我创建了一个用户表（字段：用户和密码，还有一个accounts字段连接后面的账单表）和一个账单表（用户和其他字段），
用户已经存在了，我存储账单表后，在用户表找到相应用户，在accounts字段存储对象，上面代码打印出来的却没有accounts这个字段，求大神解答谢谢哇！
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A few questions:
1. populate in mongodb can only be associated with _id;
2. You foundOne and did not execute save after modifying it, so there is no modification in the database;
3. Since it is findOne, don’t use docs in the callback, there is ambiguity;
4. It is recommended to use findOneAndUpdate directly;