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40.

DTO Input "Initializer"

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With an input DTO, the update process works like this. First ApiPlatform queries for the CheeseListing entity. Second, the JSON is deserialized into a new CheeseListingInput object. And third, our transform() method is called, where we take that CheeseListingInput object's data and move it onto the CheeseListing. To get the CheeseListing that was queried from the database, we grab it from the $context:

41 lines | src/DataTransformer/CheeseListingInputDataTransformer.php
// ... lines 1 - 9
class CheeseListingInputDataTransformer implements DataTransformerInterface
{
// ... lines 12 - 14
public function transform($input, string $to, array $context = [])
{
if (isset($context[AbstractItemNormalizer::OBJECT_TO_POPULATE])) {
$cheeseListing = $context[AbstractItemNormalizer::OBJECT_TO_POPULATE];
// ... lines 19 - 20
}
// ... lines 22 - 28
}
// ... lines 30 - 39
}

The problem with this process is step 2. Because the deserializer always creates a new CheeseListingInput with no data before putting the JSON data onto it, if a property on the input is null, we don't know if it's null because the user actually sent null for that field... or if the user simply didn't send the field at all and we should ignore it.

To fix this, before step 2, we need to somehow prepare a CheeseListingInput object that's populated with the current data from the database and tell the serializer to deserialize the JSON onto that object instead of creating a new one. If we did that, it would be safe to set everything from the input object back onto the CheeseListing because if a field was not sent, we would just be setting it to the original value from the database.

In ApiPlatform 2.6, you'll be able to do this via a new data transformer initializer. Since that's not released yet, we'll do it ourselves.

Custom Normalizer for the Input Class

How? We know that if you set the OBJECT_TO_POPULATE key on the $context, then the deserializer will use that object instead of creating a new one:

41 lines | src/DataTransformer/CheeseListingInputDataTransformer.php
// ... lines 1 - 9
class CheeseListingInputDataTransformer implements DataTransformerInterface
{
// ... lines 12 - 14
public function transform($input, string $to, array $context = [])
{
if (isset($context[AbstractItemNormalizer::OBJECT_TO_POPULATE])) {
$cheeseListing = $context[AbstractItemNormalizer::OBJECT_TO_POPULATE];
} else {
$cheeseListing = new CheeseListing($input->title);
}
// ... lines 22 - 28
}
// ... lines 30 - 39
}

By leveraging a custom denormalizer, we could hook into the denormalization process and set the OBJECT_TO_POPULATE key to a pre-populated CheeseListingInput object right before the JSON is deserialized.

If... that doesn't make sense yet, it's okay. Let's step through it piece by piece.

To start, in the src/Serializer/Normalizer/ directory, create a new CheeseListingInputDenormalizer class:

22 lines | src/Serializer/Normalizer/CheeseListingInputDenormalizer.php
// ... lines 1 - 2
namespace App\Serializer\Normalizer;
// ... lines 4 - 7
class CheeseListingInputDenormalizer implements DenormalizerInterface, CacheableSupportsMethodInterface
{
// ... lines 10 - 20
}

This will be responsible for denormalizing CheeseListingInput objects. Make it implement DenormalizerInterface and also CacheableSupportsMethodInterface, which is a performance thing:

22 lines | src/Serializer/Normalizer/CheeseListingInputDenormalizer.php
// ... lines 1 - 4
use Symfony\Component\Serializer\Normalizer\CacheableSupportsMethodInterface;
use Symfony\Component\Serializer\Normalizer\DenormalizerInterface;
class CheeseListingInputDenormalizer implements DenormalizerInterface, CacheableSupportsMethodInterface
{
// ... lines 10 - 20
}

Then go to "Code"->"Generate" - or Command+N a Mac - and select "Implement Methods" to generate the three methods we need. I'll move hasCacheableSupportsMethod() to the bottom because it's the least important:

22 lines | src/Serializer/Normalizer/CheeseListingInputDenormalizer.php
// ... lines 1 - 7
class CheeseListingInputDenormalizer implements DenormalizerInterface, CacheableSupportsMethodInterface
{
public function denormalize($data, string $type, string $format = null, array $context = [])
{
}
public function supportsDenormalization($data, string $type, string $format = null)
{
}
public function hasCacheableSupportsMethod(): bool
{
}
}

As soon as we created this class, because it implements DenormalizerInterface, the serializer will call supportsDenormalization() on every single piece of data during denormalization. In supportsDenormalization(), we support denormalizing a piece of data if its $type equals CheeseListingInput::class:

26 lines | src/Serializer/Normalizer/CheeseListingInputDenormalizer.php
// ... lines 1 - 8
class CheeseListingInputDenormalizer implements DenormalizerInterface, CacheableSupportsMethodInterface
{
// ... lines 11 - 15
public function supportsDenormalization($data, string $type, string $format = null)
{
return $type === CheeseListingInput::class;
}
// ... lines 20 - 24
}

Thanks to this, we are now 100% responsible for denormalizing CheeseListingInput objects. Down in hasCacheableSupportsMethod() return true, which you should do unless your supportsDenormalization() method uses the $context to make its decision:

26 lines | src/Serializer/Normalizer/CheeseListingInputDenormalizer.php
// ... lines 1 - 8
class CheeseListingInputDenormalizer implements DenormalizerInterface, CacheableSupportsMethodInterface
{
// ... lines 11 - 20
public function hasCacheableSupportsMethod(): bool
{
return true;
}
}

The OBJECT_TO_POPULATE during Input Denormalization

Anyways, the serializer will now call denormalize() whenever it's trying to denormalize a CheeseListingInput. Let's dump() the $context - that's the last argument - so we can see what it looks like:

26 lines | src/Serializer/Normalizer/CheeseListingInputDenormalizer.php
// ... lines 1 - 8
class CheeseListingInputDenormalizer implements DenormalizerInterface, CacheableSupportsMethodInterface
{
public function denormalize($data, string $type, string $format = null, array $context = [])
{
dump($context);
}
// ... lines 15 - 24
}

This won't work yet, but let's see what that dump looks like. At the browser - this is the put operation - hit "Execute". And... error!

Expected denormalized input to be an object

It's complaining because we're not returning anything from our denormalize() method yet... but we can check out the dump. In another tab, I already have my profiler open, click "Latest". This takes me to the "Exceptions" section. Go down and click to open the "Debug" section.

Nice! This is the $context that's being passed to denormalize. And check this out: it has an object_to_populate key set to the CheeseListing object. Well, really, that should be no surprise: we saw that a few minutes ago. Inside our data transformer, OBJECT_TO_POPULATE is set to the existing CheeseListing object:

41 lines | src/DataTransformer/CheeseListingInputDataTransformer.php
// ... lines 1 - 9
class CheeseListingInputDataTransformer implements DataTransformerInterface
{
// ... lines 12 - 14
public function transform($input, string $to, array $context = [])
{
if (isset($context[AbstractItemNormalizer::OBJECT_TO_POPULATE])) {
$cheeseListing = $context[AbstractItemNormalizer::OBJECT_TO_POPULATE];
// ... lines 19 - 20
}
// ... lines 22 - 28
}
// ... lines 30 - 39
}

But... now that I think about it... the fact that this is set to a CheeseListing object is kind of odd... because this process will ultimately deserialize the JSON into a CheeseListingInput object. How does that work?

Internally, the serializer has a sanity check: if the OBJECT_TO_POPULATE is not the same type as the object we're deserializing into, then it's ignored. That's what's happening now: API Platform sets the existing CheeseListing onto OBJECT_TO_POPULATE, but since we're not deserializing into that type of object, it's ignored and a new CheeseListingInput is created:

41 lines | src/DataTransformer/CheeseListingInputDataTransformer.php
// ... lines 1 - 9
class CheeseListingInputDataTransformer implements DataTransformerInterface
{
// ... lines 12 - 14
public function transform($input, string $to, array $context = [])
{
if (isset($context[AbstractItemNormalizer::OBJECT_TO_POPULATE])) {
// ... line 18
} else {
$cheeseListing = new CheeseListing($input->title);
}
// ... lines 22 - 28
}
// ... lines 30 - 39
}

But... we could change that key.

Setting OBJECT_TO_POPULATE to the Input DTO

Inside the denormalizer, let's start with something simple: $dto = new CheeseListingInput() and $dto->title = some hardcoded title:

30 lines | src/Serializer/Normalizer/CheeseListingInputDenormalizer.php
// ... lines 1 - 4
use App\Dto\CheeseListingInput;
// ... lines 6 - 9
class CheeseListingInputDenormalizer implements DenormalizerInterface, CacheableSupportsMethodInterface
{
public function denormalize($data, string $type, string $format = null, array $context = [])
{
$dto = new CheeseListingInput();
$dto->title = 'I am set in the denormalizer!';
// ... lines 16 - 17
}
// ... lines 19 - 28
}

Set this onto the context: $context[AbstractItemNormalizer::OBJECT_TO_POPULATE] equals $dto:

30 lines | src/Serializer/Normalizer/CheeseListingInputDenormalizer.php
// ... lines 1 - 5
use Symfony\Component\Serializer\Normalizer\AbstractNormalizer;
// ... lines 7 - 9
class CheeseListingInputDenormalizer implements DenormalizerInterface, CacheableSupportsMethodInterface
{
public function denormalize($data, string $type, string $format = null, array $context = [])
{
$dto = new CheeseListingInput();
$dto->title = 'I am set in the denormalizer!';
$context[AbstractNormalizer::OBJECT_TO_POPULATE] = $dto;
}
// ... lines 19 - 28
}

We're not done yet... but if we passed this $context into the denormalizer system, then it should deserialize the JSON onto our new object. And, whatever we return from this method will eventually be passed as the $input to our data transformer.

So next, let's finish this: by calling the denormalizer system to update the CheeseListingInput, returning it from here, proving that it's passed to the data transformer, then finally pre-filling it with database data. If you can't see how all the pieces connect yet, you will soon.