static func register(credentials: Credentials) throws -> Auth.User {
      switch credentials {
      case let credentials as UsernamePassword:
          if let user = try DemoUser.query().filter("username", credentials.username).first() {
              return user
          } else {
              return User(username: credentials.username, password: BCrypt.hash(password: credentials.password))
          }
      default:
          let type = type(of: credentials)
          throw Abort.custom(status: .forbidden, message: "Unsupported credential type: \(type).")
      }
}