Constraint annotation to validate one field depending on another field

I have this entity class in Symfony/5.4:

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;

class Assignments
{
    public const SALARY_RANGES = [
        'Red',
        'Green',
        null,
    ];

    /**
     * @ORM\Column(length=255, nullable=true)
     * @Assert\Choice(choices=Assignments::SALARY_RANGES, strict=true)
     */
    private ?string $salaryRange;

    /**
     * @ORM\ManyToOne(targetEntity="Employee", inversedBy="assignments")
     * @ORM\JoinColumn(name="employee_id", referencedColumnName="id", onDelete="CASCADE")
     */
    private ?Employee $employee;
}

I need to ensure that salaryRange has a non-null value if, and only if, employee is not null. Is it possible to enforce that with constraint annotations?

I've been playing with @Assert\Callback but I couldn't figure out how to get the value of the other field. Perhaps it's not even the right tool.

/**
 * @Assert\Callback({"ExampleValidator", "assertEmployeeOnlyCallback"})
 */

<!---->

public static function assertEmployeeOnlyCallback(mixed $data, ExecutionContextInterface $context): void
{
    // `$data` contains value from `salaryRange` but, where is `employee`?
}

just follow the documentation

https://symfony.com/doc/5.3/reference/constraints/Callback.html

class Author
{
    // ...
    private int $field = 1;
    private string $otherField;
   /**
    * @Assert\Callback
    */    
    public function validate(ExecutionContextInterface $context, mixed $payload): void
    {
        
        if ($this->field > 1 && $this->otherField != '') {
            $context->buildViolation('Your validation message')
                ->atPath('toherField')
                ->addViolation();
        }
    }
}

You can use an Expression constraint. If the expression is true, validation will pass. If the expression is false, there will be a violation.

It can be written in a single assertion, but you can write a couple of them in order to provide more accurate messages:

/**
 * @ORM\Column(length=255, nullable=true)
 * @Assert\Expression(
 *     "value !== null || this.getEmployee() === null",
 *     message="This value should not be null for employee assignments."
 * )
 * @Assert\Expression(
 *     "value === null || this.getEmployee() !== null",
 *     message="This field can only be set for employee assignments."
 * )
 * @Assert\Choice(choices=Assignments::SALARY_RANGES, strict=true)
 */
private ?string $salaryRange = null;

In my case, I also had to set a default value because I'm using a PHP typed property and it was accessed before being initialised.

This approach is as robust as using a property constraint (I presume it's just an different interface for the same underlying logic), the only drawback being that writing logic using Expression Language inside annotations is arguably harder to maintain and debug that writing pure PHP code.

The caveat mentioned in a comment about having no way to determine what order the fields are populated does not make too much sense because that is not how form validation works: reading request fields and applying validations happen in separate steps. There's no such warning in documentation either.