Datatransformer and json validation in symfony2 form - json

Question

I'm building a Rest API and I receive a json_encoded string from the clients.
I want this string to be decoded before saving my entity, because it's going into a jsonb field in PostgreSQL.
The behavior I want is :
Validate that the string is valid json, if not, add a violation in the form via a custom validator
Automatically decode the string and set the json object in the entity property
I've tried two different strategies
In the entity setMetadata($value) method, if $value is a string, I decode it
I created a DataTransformer that json_decode the value received in the form
But both these solutions don't work because the custom validator I created is called after, and it calls directly $lesson->getMetadata(). Since the value has already been decoded (either in the setMetadata() method or in the DataTransformer, the validator receive either a json object or null. So I can't add a violation to the form, since I have no way to know if the value received was actually null, or if the string was malformed.
Here is the lesson entity:
class Lesson extends BaseContent
{
[…]
/**
* #var jsonb
*
* #ORM\Column(name="metadata", type="jsonb", nullable=true)
* #KreactiveAssert\Json
*/
private $metadata;
[…]
}
Here is the custom validator:
class JsonValidator extends ConstraintValidator
{
public function validate($value, Constraint $constraint)
{
if ($value && !json_decode($value)) {
$this->context->addViolation($constraint->message, array('%string%' => $value));
}
}
}
And here is the DataTransformer:
class StringToJsonTransformer implements DataTransformerInterface
{
/**
* Transform a json object to a string
* #param Json|null $json
* #return String
*/
public function transform($json)
{
if (null === $json) {
return "";
}
return json_encode($json);
}
/**
* Transform a string to a json object
* #param String $string
* #return Object
*/
public function reverseTransform($string)
{
if (!$string) {
return null;
}
throw new TransformationFailedException('error transforming');
return json_decode($string);
}
}
Is there any way I can validate the input data in the form, and then set the metadata as a json object?

Answer 1

I've found this (I don't know how come I didn't find it earlier):
Combine constraints and data transformers
I'm going to make an ugly workaround as suggested, even though I don't like that solution.
<?php
class StringToJsonTransformer implements DataTransformerInterface
{
/**
* Transform a string to a json object
* #param String $string
* #return Object
*/
public function reverseTransform($string)
{
if (!$string) {
return null;
}
/*
* UGLY WORKAROUND
* we return -1 if the json_decode fail
* so the validator can add a violation in the form telling
* the json string was not valid
* If we don't do this, the validator will receive either
* null or a json object. In case of null, there is no way to
* tell if the client sent null, or if the decoding failed
*/
$value = json_decode($string);
return $value ? $value : -1;
}
}
I'm still not sure if I'm going to return -1 or something else. In the custom validator, I get an error if I try to compare a jsonObject with -1 (which is normal).