Use a function in a Mongoose query - mongodb

Question

I have a basic schema that implements getters/setters:
var User = new Schema( {
username : { type : String , required : true , get: getUsername, set: setUsername }
} );
User.set('toObject', { getters: true , virtuals : true } );
User.set('toJSON', { getters: true , virtuals : true } );
UserModel = mongoose.model('User', User);
setUsername is fairly simple but "randomizes" the username, and obviously getUsername retrieves the original value. Now, everytime I call setUsername, I will get a different "random" string (I mean it's pretty much a hash function).
My question is, can I somehow use the getter in a query?
Like
UserModel.$where( 'this.username.get() == "<somestring>"' ).exec( function( err , users ) {
// do something with the users
}
} ); // here I get MongoError: TypeError: this.username.get is not a function
or
UserModel.find( { $where: function() {
return( getUsername( this.username ) == "<somestring>" )
} } , function( err , users ) {
// do something with the users
} ); // here I get MongoError: ReferenceError: getUsername is not defined
I tried adding a getUsername method to the schema as well, adding virtuals, etc., but I cannot seem to find a solution. i've tried about 15 different syntaxes but I either get an error, an empty array or all users.
Basically if I have a user whose username is "a", it'll be saved in my DB as *"b", "c", "d", or whatever else the setUsername function sets it to.
I'd like to retrieve the user knowing only that their username is "a".
Thank you!

Answer 1

Yes, this should theoretically be possible using query middleware, but you might have to think a little differently:
Let's say you want to run a query like this:
UserModel.find({user: 'a'})
where 'a' is the unscrambled input and you actually want to automatically lookup {user: getUser('a')}, which we'll say is {user: 'b'}
Then you could use this middleware to modify your Query like this:
const userFilter = function (next) {
const query = this.getQuery();
if (query.userName) {
// this transforms the query in-place from {user: 'a'} to {user : 'b'}
query.userName = setUserName(query.userName)};
}
return next();
};
// let's hook this one in!
User.pre('find', userFilter);
Note: This will hook on all queries including ".userName" If you only use UserModel.find() then this will suffice, but otherwise you might also want to register this hook on ['count', 'find', 'findOne', 'findOneAndRemove','findOneAndUpdate', 'remove', 'update', 'updateOne', 'updateMany'] and that some queries (iirc e.g. findByIdAndUpdate) is supposed to bypass mongoose and directly use the mongodbDriver, which might also bypass this hook... Even though it still seems to work currently, so you might want to test all kinds of functions with it)
EDIT:
Looking back at your question, I am not sure, if I understood your question correctly. What does the setUsername function actually return? Is it always the same output for the same input? If so, you basically always to query {userName: setUserName('a')}, right?