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03.

Decorating Data Persisters vs Context Builders

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To do the actual saving of the User, we want to call the specific core ApiPlatform data persister that normally saves entities. So... how can we do that?

To start, we need to figure out which service is responsible for that. Let's do some digging: run bin/console debug:container and search for api_platform.

php bin/console debug:container api_platform

This shows all of the services from ApiPlatform... which is a bunch. Search for "doctrine". Ok: a metadata factory and... ah: api_platform.doctrine.orm.data_persister. I'm pretty sure that's what we want. I'll enter the number next to it - 105 - to get more details.

Ok, the class is called just DataPersister. Back in PhpStorm, hit Shift+Shift, search for DataPersister and make sure to include "non-project items". Select the one from ApiPlatform's Doctrine Bridge.

And... cool! This looks pretty much exactly like we expected: it persists and flushes. This is what we want to inject into our data persister.

Injecting the Doctrine Persister

Let's do it! Back at the terminal, copy the service id. Instead of relying on autowiring, we can explicitly configure this argument on UserDataPersister. To do that, open config/services.yaml, head to the bottom, and override the service decoration for App\DataPersister\UserDataPersister. Beneath, add bind and then the argument name, which is decoratedDataPersister. So $decoratedDataPersister set to an @ symbol and then the service id:

43 lines | config/services.yaml
// ... lines 1 - 7
services:
// ... lines 9 - 39
App\DataPersister\UserDataPersister:
bind:
$decoratedDataPersister: '@api_platform.doctrine.orm.data_persister'

Thanks to this, Symfony knows to inject this specific service instead of the normal chain data persister. To prove it, try the tests again:

symfony run bin/phpunit --filter=testCreateUser

And... got it!

Class Decoration vs Service Decoration

What we just did got pretty deep into ApiPlatform, Symfony and services... but this is the kind of stuff that we're going to be doing in this tutorial.

And, I want to point out a subtle difference between what we just did and how we hooked into a different part of ApiPlatform in the last tutorial. Specifically, the context builder system. In both cases, we wanted to run custom code inside part of ApiPlatform. For AdminGroupsContextBuilder, we used a decorates option:

43 lines | config/services.yaml
// ... lines 1 - 7
services:
// ... lines 9 - 32
App\Serializer\AdminGroupsContextBuilder:
decorates: 'api_platform.serializer.context_builder'
arguments: [ '@App\Serializer\AdminGroupsContextBuilder.inner' ]
// ... lines 36 - 43

But in this case, we didn't. Why?

How the Context Builder System Works

Let's talk about how the context builder system works in Symfony's service container. Open up our custom class: App\Serializer\AdminGroupsContextBuilder:

37 lines | src/Serializer/AdminGroupsContextBuilder.php
// ... lines 1 - 8
final class AdminGroupsContextBuilder implements SerializerContextBuilderInterface
{
// ... lines 11 - 19
public function createFromRequest(Request $request, bool $normalization, ?array $extractedAttributes = null): array
{
$context = $this->decorated->createFromRequest($request, $normalization, $extractedAttributes);
$context['groups'] = $context['groups'] ?? [];
$isAdmin = $this->authorizationChecker->isGranted('ROLE_ADMIN');
if ($isAdmin) {
$context['groups'][] = $normalization ? 'admin:read' : 'admin:write';
}
$context['groups'] = array_unique($context['groups']);
return $context;
}
}

The class itself is pretty simple: it implements SerializerContextBuilderInterface and adds some conditional serialization groups based on the current user. We also inject a $decorated object that has the same SerializerContextBuilderInterface:

37 lines | src/Serializer/AdminGroupsContextBuilder.php
// ... lines 1 - 8
final class AdminGroupsContextBuilder implements SerializerContextBuilderInterface
{
private $decorated;
private $authorizationChecker;
public function __construct(SerializerContextBuilderInterface $decorated, AuthorizationCheckerInterface $authorizationChecker)
{
$this->decorated = $decorated;
$this->authorizationChecker = $authorizationChecker;
}
// ... lines 19 - 35
}

So, on a high-level, this is "class decoration": we are decorating some other serializer context builder, calling it, but also adding our own logic. In UserDataPersister, we are also doing class decoration in the exact same way.

2 Systems: Different Extension Points

But... the way we get this working in Symfony's service config is different. Why? Because the way these systems work in ApiPlatform is different, requiring two distinct solutions.

Check this out, run:

php bin/console debug:container context

One of the results is a service called api_platform.serializer.context_builder. This is the one context builder service in ApiPlatform. Or, to say that differently, when ApiPlatform needs to "build the context" at the beginning of the request, it calls this one service to do that job.

Copy the service id and re-run debug:container with that id and --show-arguments:

php bin/console debug:container api_platform.serializer.context_builder --show-arguments

Perfect! Notice the name: SerializerFilterContextBuilder. I don't know what that is, but it does not sound like a "chain" context builder. And check out that third argument: it's passed our AdminGroupsContextBuilder service!

Here's what's going on. Inside ApiPlatform, there is only one context builder service. But unlike the "data persister" system, this one context builder is not a "chain" class that calls many other context builders. Nope, there is simply just one context builder service... and that's it.

Service Decoration: Services inside Services

That creates a problem: if we want to add logic to the context builder service, the only way we can do that is by replacing the core service entirely. That is the job of Symfony's service decoration, and ApiPlatform relies on it in several different places.

Yep, the decorates option basically says this:

I want to replace the core context builder service. I literally want to become the service that you get when someone asks for the api_platform.serializer.context_builder service. But, I also want that original context builder object to be passed to me as an argument.

What you ultimately get is a bit like a Matryoshka doll, where you have one context builder service, with another inside and another inside it! In this case, the "outer" context builder is apparently a SerializerFilterContextBuilder. But then, it does its work and calls our AdminGroupsContextBuilder.

And if you run this same debug:container command on our service:

php bin/console debug:container App\Serializer\AdminGroupsContextBuilder --show-arguments

You'll see that our service is passed another one with the same name, but .inner. If you debug that:

php bin/console debug:container App\Serializer\AdminGroupsContextBuilder.inner --show-arguments

Ah, its class is SerializerContextBuilder! This is the true, original context builder service. It's been decorated twice - once by us and once by ApiPlatform itself - to add more features.

The point is: by leveraging service decoration, we can create a, sort of, "chain" of context builders... even though there isn't technically a chain class. But for the data persister system, this... simply isn't necessary! That system has a "chain data persister" and we can freely add however many data persisters we want simply by creating a class that implements DataPersisterInterface. Well, technically, we need to create a service and tag it with api_platform.data_persister... but that happens automatically thanks to auto-configuration.

The point is, thanks to how the data persister system is built in ApiPlatform, we didn't need to replace the core data persister: we could just add our's to the chain. If we want to call one of the other data persisters, we can inject it. But it's not service decoration: we're not replacing that service.

To me, the data persister system is a better design: it makes life a lot simpler. Service decoration is a bit more confusing, but it is a valid option when... it's the only option, like in the case of context builders. That is why you see two different solutions to "hooking into" a core part of ApiPlatform.

Next: the super power of a custom data persister is the ability to do something before or after an entity is saved. But what if the code we need to run should only be called when an object is created? Or updated? Or only for a specific operation? Let's find out how we can do that!