How to force already registered user to login during checkout in Sylius API?

You can force the user to log in during checkout if he is already registered in your app.

Create a new constraint validator

Firstly you need to add a new constraint class:

// src/Validator/Constraints

<?php

declare(strict_types=1);

namespace App\Validator\Constraints;

use Symfony\Component\Validator\Constraint;

final class UserAlreadyRegistered extends Constraint
{
    public string $message = 'This email is already registered. Please log in.';

    public function validatedBy(): string
    {
        return 'sylius_api_registered_user_validator';
    }

    public function getTargets(): string
    {
        return self::CLASS_CONSTRAINT;
    }
}

Then you need to add a validator class:

// src/Validator/Constraints

<?php

declare(strict_types=1);

namespace App\Validator\Constraints;

use Sylius\Component\Core\Model\CustomerInterface;
use Sylius\Component\Core\Model\ShopUserInterface;
use Sylius\Component\Core\Repository\CustomerRepositoryInterface;
use Symfony\Component\Security\Core\Authentication\Token\Storage\TokenStorageInterface;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;
use Webmozart\Assert\Assert;

final class UserAlreadyRegisteredValidator extends ConstraintValidator
{
    public function __construct(
        private CustomerRepositoryInterface $customerRepository,
        private TokenStorageInterface $tokenStorage
    ) {
    }

    public function validate($value, Constraint $constraint): void
    {
        /** @var UserAlreadyRegistered $constraint */
        Assert::isInstanceOf($constraint, UserAlreadyRegistered::class);

        $token = $this->tokenStorage->getToken();

        /** @var CustomerInterface|null $existingCustomer */
        $existingCustomer = $this->customerRepository->findOneBy(['email' => $value->getEmail()]);
        if (null !== $existingCustomer && !$token->getUser() instanceof ShopUserInterface) {
            $this->context->addViolation($constraint->message);
        }
    }
}

Enabling your validator

As the last thing you need to do is enable this constraint validator in your app and register it as a service in services.yaml. You can do it this way:

// # config/validator/validation.xml

<?xml version="1.0" encoding="UTF-8"?>

<constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping http://symfony.com/schema/dic/services/constraint-mapping-1.0.xsd">
    <class name="Sylius\Bundle\ApiBundle\Command\Checkout\UpdateCart">
        <constraint name="App\Validator\Constraints\UserAlreadyRegistered">
            <option name="groups">
                <value>sylius</value>
            </option>
        </constraint>
    </class>
</constraint-mapping>

// # config/services.yaml

services:
    # other definitions
    App\Validator\Constraints\UserAlreadyRegisteredValidator:
        class: App\Validator\Constraints\UserAlreadyRegisteredValidator
        tags: [ { name: validator.constraint_validator, alias: sylius_api_registered_user_validator } ]