域驱动的设计可以重视对象的引用/嵌入实体

Question

I have a small issue understanding ddd value objects, so a couple questions has raised from my domain that i really need help solving them,

First question :

Lets say i have a car entity and wheel value object the wheel has a two entities which are rubber and rims so the issue is the wheel allowed to have entities inside it?

example :

namespace Domain\ValueObjects;

class Wheel{

    private $rim;

    private $rubber;

    public function __construct(Domain\Models\Rim $rim, Domain\Models\Rubber $rubber){
        $this->rim = $rim->toArray();
        $this->rubber = $rubber->toArray();
    }
}

Second question :

when i map the car from my infrastructure to an actual domain model how im gonna map the rim if i wanna say return the wheel rim, in another word how im gonna construct an object of type rim from the wheel ? is it a static function like public static function fromNative() ;

Note :

to be more specific about question two as you may notice when im constructing the value objects, im setting the object as native type which is array so i remove mapping responsibility from the ODM , is that a good practice or should i leave that responsibility to ODM .

Answer 1

In response to your first question, value objects represent an object which has no identity and is only comparable by it's value. Entities have identity, so even if their values are the same, they are still different objects fundamentally. So in your scenario, once you include entities within a value object, then you no longer have a value object since you can't compare it based on value alone.

If rim and rubber are value objects however, then it's fine to have a composite wheel value object.

For the second question, if you are asking how to create an entity which has a value object from within your data layer, then I suggest you do if from the ODM if possible. Sometimes custom mapping may be required depending on how complex it is. I would definitely avoid any methods which serve infrastructure concerns to be added to the domain objects.