How to populate model ref property after instantiation?

manual-populate.js

var commentsSchema = new Schema({
    content: { type: String },
    _user: { type: Schema.ObjectId, ref: 'users' },
});


//...


// Get a comment without populating
CommentsModel
.findOne({ _id: COMMENT_ID }, function(err, comment) {

    UsersModel
    .findOne({ _id: comment._user }, function(err, user) {
        
        // Attempt to set user model on comment (like .populate)
        comment.set('_user', user);
        console.log(comment);
        //=> comment._user is still just the original object id. 
        // .set() command seems to have been ignored by Mongoose.
        
    })
});

If the comment._user was not populated on the original find, what is the best method to mimic populate()? How can I force mongoose to set the user model on the comment._user property? I think it's something to do with types, not sure how to work round it though.