DDD journey: how many value objects should we use?

Foreword

In the series of my short DDD journey articles, I’ll summarize some of the insights that came out speaking with the author of the Advanced Web Application Architecture book.

Thanks to the author, you can get it with a special discount reserved to the readers of this blog!

TLDR: the more, the better

One thing I started doing after reading Noback’s book was using value objects.

Using the words of Martin Fowler, value objects are a way to represent things as a compound.

Using value objects instead of scalars for class property types has some benefits.

In fact, we can encapsulate validation logic inside the constructors of value objects.

Say that we need an email property in a User class. Instead of using a string type for the email, we create a value object like this:

final class Email
{
    private string $address;

    private function __construct(string $address)
    {
        $emailRegEx = 'TODO: your preferred email validation regex';
        if (0 === preg_match($emailRegEx, $address)) {
            throw new InvalidArgumentException('invalid email address');
        }

        $this->address = $address;
    }

    public static function fromString(string $string)
    {
        return new self($string);
    }

    public function asString()
    {
        return $this->address;
    }
}

Using the Email type in the User class allows us to protect its internal consistency. In other words, we don’t have any way to instantiate a User object with an invalid email.

The more value objects we use, the better.

Enjoy!

Posted with : ddd