User.get({username: username})
    .then((retrievedUser) => {
      if (retrievedUser) {
        if (retrievedUser.checkPassword(password)) {
          return retrievedUser;
        }
        throw new AuthError('A user already exists, but your password did not match. Please check your password.');
      }
      return User.create({username: username, password: password});
    })
    .then((authedUser) => {
      setAsCurrentUser(authedUser);
      redirect('/');
    })
    .catch((error) => {
      if (error instanceof AuthError) {
        redirect('/login');
      } else {
        send500();
      }
    });

User.get({username: username})
    .then((retrievedUser) => {
      if (retrievedUser) {
        throw new UserExistsError();
      }
      return User.create({username: username, password: password});
    })
    .then((authedUser) => {
      setAsCurrentUser(authedUser);
      redirect('/');
    })
    .catch((error) => {
      if (error instanceof UserExistsError) {
        redirect('/login');
      } else {
        send500();
      }
    });