title: How to validate whether a username is unique using joi?
body: <p>I read an article somewhere in which the author used <a href="https://github.com/hapijs/joi" rel="noreferrer">Joi</a> to validate asynchronously, whether the username is unique or not by checking with the database. I can't find it now and I want to know how can we do that with <code>Joi</code>.</p>