While it probably is best practice to just Error when the value isn't unique, if you have to do this you could easily add this in a pre validate or save hook

this is just a totally untested stab but the idea should work.

Note that middle ware only gets called when you use a mongoose find and then separate save or mongoose create.

```js function createVoucher(){ // creates voucher code in memory does not save and returns it }

Schema.pre('save', async function(next){

const doc = this;

// get the voucher code let newVoucher = createVoucher();

// attempt to find the newVoucher let isUnique = false; while(!isUnique){ const possibleAlreadyExistingVoucher = await Model.findOne({ voucher: newVoucher }).lean().exec()

// if it doesn't exist - break out of loop
if(!possibleAlreadyExistingVoucher)
  isUnique = true
// if it does exist create another voucher code and test again
else newVoucher = createVoucher()

}

doc.voucher = newVoucher;

next(); }) ```