static func authenticate(credentials: Credentials) throws -> Auth.User {
    switch credentials {
    case let id as Identifier:
        guard let user = try User.find(id.id) else {
            throw Abort.custom(status: .forbidden, message: "Invalid user identifier.")
        }
        return user

     case let usernamePassword as UsernamePassword:
          let fetchedUser = try User.query().filter("username", usernamePassword.username).first()
          guard let user = fetchedUser else {
                throw Abort.custom(status: .networkAuthenticationRequired, message: "User does not exist")
          }
          if try BCrypt.verify(password: usernamePassword.password, matchesHash: fetchedUser!.password) {
              return user
          } else {
              throw Abort.custom(status: .networkAuthenticationRequired, message: "Invalid user name or password.")
          }

     case let credentials as FacebookAccount:
          if let existing = try DemoUser.query().filter("facebook_id", credentials.uniqueID).first() {
              user = existing
          } else {
              throw Abort.custom(status: .networkAuthenticationRequired, message: "User does not exist")
          }

     case let credentials as GoogleAccount:
          if let existing = try DemoUser.query().filter("google_id", credentials.uniqueID).first() {
              user = existing
          } else
              throw Abort.custom(status: .networkAuthenticationRequired, message: "User does not exist")
          }

     default:
          let type = type(of: credentials)
          throw Abort.custom(status: .forbidden, message: "Unsupported credential type: \(type).")
     }
}