Add field value by Constructor.count in pre validate hook in mongoose

Question

My Schema username is set to unique & required to false

on creating user I don't include username!

when I try to create a second user it throws error on validate because the first user has username set to null and it is a duplicate to second user's username field.

So, I tried setting the username to total number of users in collection but Count() runs in last and also does not update the username

UserSchema.pre('validate', function(next) {

const self = this;

// count() runs after pre.save()

self.constructor.count(async function(err, data) {
     var x = data
     self.username = "name"+data
    console.log(data)
});

// this console outputs before constructor.count()

console.log(this.username)
next()

});

// even self.set() returns Set is not a function inside Constructor.Count

I tried running constructor.count() through variable function so it runs before setting the username and retrieve the total number of collection from count but it still doesn't work