Mongoose dbref problem

  node.js, question
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);
 });
 });

I created a user table (fields: user and password, and an accounts field connecting the following bill tables) and a bill table (user and other fields).
The user already exists. After I stored the bill table, I found the corresponding user in the user table and stored the object in the accounts field. The code printed above does not have the accounts field. Please ask the great god for an answer. Thank you!

Several questions:
1. mongodb inside populate can only be associated with _id;
2. You findOne and did not execute save after modification, so the database inside has not been modified;
3. Since it is findOne, do not use docs when calling back inside, which is ambiguous.
4. It is suggested to use findOneAndUpdate directly.