Symfony 2中的DDD-如何与Validator服务进行交互？

Question

Some time ago I've started playing with DDD. For now, all my classes are POPO. Almost all is complete but I'd like to validate my entities before persistence.

I already know where to put validation (commands/use case objects) but ideally I would like to use a validator service in application/infrastructure layer. For me, validation should be part of domain layer but if I put it there, I'll have a lot of duplication.

Have you encountered such problem? Is there any reasonable solution for that? Thanks for all!

Answer 1

Don't mix the type of validations, I know it confuses and is not easy to separate concepts and do it the right way, but don't give up.

There are business validations which should belong to your domain and don't use any external component/framework to validate (since your domain should be the most clean and natural possible), and application validations, which should check whether a URL is valid, if user is authenticated, and you could use Symphony or whatever framework/plugin you want.

Your domain should be the most natural and clean possible, in a way you could even show it to a business analyst and discuss things. The validations you put inside your domain should follow this concept, which means that business analyst could even point to you if that validation is correct, and suggest changes if it's the case. Actually where I work, sometimes even the Key Users see the Domain (code and diagrams) and point out things.

Now suppose anyway you do want to validate whether an URL is valid inside your domain, and don't follow the Always Valid approach (which is recommended), where you would assume the application layer already validated the URL for you. Instead, you do want to have a line of code inside your domain explicitly trying to validate an URL. If this is a requirement (and I'm against, but I'm giving a possible solution for it), I'd do this:

// This would belong to your domain...
interface IValidator
{
    public function IsValidURL($url);
}

class Foo
{
    public function SaveURL($url, $validator)
    {
        if (!$validator instanceof IValidator)
            throw new Exception("Invalid validator providen to Foo!");
        if (!$validator->IsValidURL($url))
            throw new Exception("The URL $url is not valid!");
        // Do logic
    }
}

// ...and this to your Application Layer
class SymphonyValidator implements IValidator
{
    public function IsValidURL($url)
    {
        // use Symphony validator or any other framework/plugin
    }
}


var foo = new Foo();
var validator = new SymphonyValidator();
foo->SaveURL("invalidUrl", validator);

But keep in mind this is just a workaround to call a validation method inside your domain, injecting a third-party component to do the hard job. I'm personally against this, but if someone asks you "where do you guarantee the URL is valid? I can't find it in your domain" you could do this way (I'd argue URL validation is not up to business, but to infra/app layers)

Actually, the domain would still not guarantee the URL is valid, because the validation is still provided by the application layer (what if the Validator class always returned true?), so that's why I'm against this. If you need this anyway, at least if you change from symphony to any other kind of validator, your domain is kept untouched and your domain is not coupled to specific third-party libraries, and any application could provide its own Validator (since your domain can be used by may applications like web, mobile, desktop, external API calls, etc).