未检测到Symfony / Doctrine ORM映射

I'm building my first Symfony bundle. I want to use Models instead of Entity so I have a directory Model which contains only one model. Also I have a mapping for this model which is located in Resources/config/doctrine/mymodel.orm.yaml. When I run /bin/console doctrine:migration:diff it says No changes detected in your mapping information

Here's is my model

class MyModel
{
    /**
     * @var mixed
     */
    private $id;

    /**
     * @var string
     */
    private $user;

  /**
     * @return mixed
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * @return null|string
     */
    public function getUser(): ?string
    {
        return $this->user;
    }

    /**
     * @param string $user
     *
     */
    public function setUser(string $user): self
    {
        $this->user = $user;

        return $this;
    }

and the mymodel.orm.yaml

MyNamespace\MyBundle\Model\MyModel:
    type: mappedSuperclass
    repositoryClass: MyNamespace\MyBundle\Repository\MyModelRepository
    id:
        id:
            type: integer
            generator:
                strategy: AUTO
    fields:
        user:
            type: string

What I'm missing here?

UPDATE

I'm using symfony 4.2.3

Here's my doctrine.yaml

parameters:
    env(DATABASE_URL): ''

doctrine:
    dbal:
        driver:
        charset: utf8
        default_table_options:
            charset: utf8
            collate: utf8_unicode_ci

        url: '%env(resolve:DATABASE_URL)%'
    orm:
        auto_generate_proxy_classes: '%kernel.debug%'

        entity_managers:
            default:
                auto_mapping: true
                mappings:
                    MyBundle: ~