Mongoose Caching a Part of a Referenced Subdoc on Creation - mongodb

Question

I have a simple schema with references. But I dont want to populate the referenced document on every query. Instead I want to cache a part (only a single attribute) on the referencing document.
A simple example schema:
User
- displayName
(....stuff....)
Posts
- title
- content
- user (reference?)
When I used references and population on demand my PostSchema would look like:
var PostSchema = new Schema({
...
user: {
type: Schema.ObjectId,
ref: 'User'
}
})
But I want to save (cache) only the displayName (not the other stuff in User) and the _id in Post when it is created. My thougts on that is to change the PostSchema as following:
user: {
_id: {
type: Schema.ObjectId,
ref: 'User'
},
displayName: String
}
on the creation of a post I do:
var post = new Post(req.body);
post.user._id = req.user._id;
post.user.displayName = req.user.displayName;
The problem with this is: It looks like now _id is the referenced user, leading to absurd user._id._id. Also my isCreator-middleware now needs to convert the _id to a string:
exports.isCreator = function(req, res, next) {
if (req.post.user._id.toString() !== req.user._id.toString()) {
return res.status(403).send('User is not authorized');
}
next();
};
This cannot be the best solution. So my question is: What is best practice for this case?