01-IdentifiesAggregate.php

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01-IdentifiesAggregate.php 02-DomainEvent.php 03-RecordsEvents.php 04-ProtectsInvariants.php 05-ProtectsMoreInvariants.php 06-IsEventSourced.php 07-EventStore.php 08-AggregateRepository.php index.markdown

01-IdentifiesAggregate.php
¶
```
<?php
```
¶

We use a Value Object to identify an Aggregate. This prevents the sort of bugs where id’s got mixed up, and is a nice and explicit way of dealing with id’s anyway. You can use any kind of id’s, as long as they implement IdentifiesAggregate. UUIDs are recommended, because they are universally unique, and they can be generated in the client. Some infrastructure, such as an Event Store, might require UUIDs exclusively, so they can optimize for it.
```
namespace Buttercup\Protects\Tests;
use Buttercup\Protects\IdentifiesAggregate;
```
¶

Simply use the IdentifiesAggregate interface on a class such as BasketId, and implement its methods.
```
final class BasketId implements IdentifiesAggregate
{
    private $basketId;
```

You are free to extend from an abstract class, and to implement the constructor as you wish. For example, you could add some validation in there.

    /**
     * @param string $basketId
     */
    public function __construct($basketId)
    {
        $this->basketId = (string) $basketId;
    }

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

    public function __toString()
    {
        return $this->basketId;
    }

    public function equals(IdentifiesAggregate $other)
    {
        return
            $other instanceof BasketId
            && $this->basketId == $other->basketId;
    }

¶

A nice convention is to have a static generate() method to create a random BasketId. The example here is a bad way to generate a UUID, so don’t do this in production. Use something like https://github.com/ramsey/uuid
```
    public static function generate()
    {
        $badSampleUuid = md5(uniqid());
        return new BasketId($badSampleUuid);
    }
}
```

Sample usage:

$basketId = BasketId::fromString('12345678-90ab-cdef-1234-567890abcedf1234');

Casting to string gives you the original string back:

it("should cast to string",
    (string) $basketId == '12345678-90ab-cdef-1234-567890abcedf1234');

Testing equality:

it("should equal instances with the same type and value",
    (new BasketId('same'))->equals(new BasketId('same')));
it("should not equal instances with a different value",
    !(new BasketId('other'))->equals(new BasketId('same')));