Useful

schema.pre('validate', function(next) {
    var url = this.url;
    if (!url)
        url = '/' + this.title;

    url = url.replace(/[\?\'\"\@\!\#\$\%\^\&\*\(\)\+\=\_\~\{\}\[\]\\\|\,\;\:]/g, "")
        .replace(/ +/g, "-")
        .replace(/\-+/g, '-')
        .replace(/(?:^\-|\-$)/g, '');

    if (url.substr(0,1) !== '/')
        url = '/' + url;

    this.url = url.toLowerCase();
    next();
});