Amplify: Unique Username Check?

[open matrix]

https://dev.to/callmekatootie/aws-appsync-with-amplify-how-to-ensure-a-field-in-a-type-is-unique-51hp
This tutorial is 2 years out of date, and I can't figure it out. How are we supposed to do a Unique Username Check in Amplify now? Assume that I want users to be able to change their username at any time, and potentially add other username filters (such as minimum length, or character restrictions).

Currently I have email auth and Google Oauth. I am storing a Username as a separate model field.

[strange bloom]

you can use a lambda resolver and do the filtering in there