mongoose dout

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

    // set code automatically

    // generate all possible codes
    // this is a fixed list from A/001 to Z/100
    let _all_codes = new Set()
    for (let _letter of 'ABCDEFGHIJKLMNOPQRSTUVWXYZ') {
        for (let i = 1; i <= 100; i++) {
            _all_codes.add(_letter + '/' + sprintf('%03d', i));
        }
    }

    // get all codes from db
    var _db_codes = new Set();

    mongoose.models['Client']
        .find()
        .select('-_id code')
        .sort('code')
        .lean()
        .exec(function(err, codes) {
            if (err) {
                console.log(err)
            }
            
            //
            
            console.log(_all_codes)
        })

    // get next code
    // *1 return the first gap and set the code 
    // this.code = 'D/078'
    
    next();
})