Symfony serialization: changing default DateTimeNormalizer format

Question

I am trying to serialize an entity class which has some \DateTime fields. Everything works fine, but \DateTime objects are converted to string using the following format: "2019-10-21T01:05:12+00:00", while I would like to get just the date part: "2019-10-21".

Symfony documentation mentions default format but doesn't explain how to configure it:

DateTimeNormalizer This normalizer converts DateTimeInterface objects (e.g. DateTime and DateTimeImmutable) into strings. By default, it uses the RFC3339 format.

Is it possible to change the default DateTime normalization format and how?

Entity class:

class Fact
{
    /**
     * @ORM\Column(type="datetime", options={"default": "CURRENT_TIMESTAMP"})
     * @Groups({"api"})
     */
    private $created_on;
}

Normalization example:

use Symfony\Component\Serializer\SerializerInterface;

class FactController extends AbstractController
{
    private $serializer;

    public function __construct(SerializerInterface $serializer)
    {
        $this->serializer = $serializer;
    }

    public function view($id)
    {
        ....   
        $data = array(
            'fact' => $this->serializer->normalize($fact, null, ['groups'=> 'api']),
        );
        ...
    }
}

Answer 1

It turns out that '$context' array is passed down to normalize() function of each supported Normalizer. Built-in Normalizers define array keys they accept and their default values.

The relevant key in my case is 'datetime_format', which defaults to \DateTime::RFC3339. Format must be the one accepted by \DateTime::format() and \DateTime::createFromFormat() methods - these functions are used for normalization / denormalization.

Correct usage in my case is:

    public function view($id)
    {
        ....   
        $data = array(
            'fact' => $this->serializer->normalize($fact, null, ['groups'=> 'api',
                                                                 'datetime_format' => 'Y-m-d']),
        );
        ...
    }

Answer 2

Here is an answer in another question. Solution is:

services:
   serializer.normalizer.datetime:
       class: ‘Symfony\Component\Serializer\Normalizer\DateTimeNormalizer
       arguments
           -
               !php/const Symfony\Component\Serializer\Normalizer\DateTimeNormalizer::FORMAT_KEY: 'Y-m-d\TH:i:s.uP’
       tags:
           - { name: serializer.normalizer, priority: -910 }

Priority is taken from the original service, so, this solution will not have any side effects.