Shopware 6: Create inherited entity extension for products

I want to create a product extension in Shopware 6.3.5.4 and I followed this guide. It should be a OneToOne Association and I want to fill this extension only for parent products, because the data in there is the same for all variants of a product. So if a variant is loaded it should load the extension of the parent.

This is my Extension:

class MyExtension extends EntityExtension
{
    public const EXTENSION_NAME = 'myExtension';

    public function extendFields(FieldCollection $collection): void
    {
        $collection->add(
            (new OneToOneAssociationField(
                self::EXTENSION_NAME,
                'id',
                'product_id',
                MyExtensionDefinition::class,
                true
            ))->addFlags(new Inherited())
        );
    }

    public function getDefinitionClass(): string
    {
        return ProductDefinition::class;
    }
}

This is the defineFields function of my extension definition:

protected function defineFields(): FieldCollection
{
    return new FieldCollection([
        (new IdField('id', 'id'))->addFlags(new Required(), new PrimaryKey()),
        new VersionField(),

        (new StringField('material_1', 'material1')),
        (new StringField('material_2', 'material2')),
        (new StringField('material_3', 'material3')),
        (new StringField('material_4', 'material4')),

        (new FkField('product_id', 'productId', ProductDefinition::class))->addFlags(new Required()),
        (new ReferenceVersionField(ProductDefinition::class))->addFlags(new Required()),
        new OneToOneAssociationField('product', 'product_id', 'id', ProductDefinition::class, false),
    ]);
}

This is my extension entity:

class MyExtensionEntity extends Entity
{
    use EntityIdTrait;

    protected string $productId;
    protected string $productVersionId;
    protected ?ProductEntity $product = null;
    protected ?string $material1 = null;
    protected ?string $material2 = null;
    protected ?string $material3 = null;
    protected ?string $material4 = null;

    // getters and setters
}

And finally my migration:

class Migration1622484776MyExtension extends MigrationStep
{
    use InheritanceUpdaterTrait;

    public function getCreationTimestamp(): int
    {
        return 1622484776;
    }

    public function update(Connection $connection): void
    {
        $sql = <<<SQL
            CREATE TABLE IF NOT EXISTS `my_extension` (
                `id`                        BINARY(16)      NOT NULL,
                `version_id`                BINARY(16)      NOT NULL,
                `material_1`                VARCHAR(255)    NULL,
                `material_2`                VARCHAR(255)    NULL,
                `material_3`                VARCHAR(255)    NULL,
                `material_4`                VARCHAR(255)    NULL,
                `product_id`                BINARY(16)      NOT NULL,
                `product_version_id`        BINARY(16)      NOT NULL,
                `created_at`                DATETIME(3)     NOT NULL,
                `updated_at`                DATETIME(3)     NULL,
                PRIMARY KEY (`id`, `version_id`),
                CONSTRAINT fk_my_extension__product 
                    FOREIGN KEY (`product_id`,`product_version_id`) REFERENCES `product` (`id`,`version_id`) ON DELETE CASCADE ON UPDATE CASCADE
            ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
SQL;
        $connection->executeUpdate($sql);

        $this->updateInheritance($connection, 'product', MyExtension::EXTENSION_NAME);
    }

    public function updateDestructive(Connection $connection): void
    {
        // implement update destructive
    }
}

After I run an import script which imports products and the extensions it looks fine in the database. There are only extensions for parent products and in the product table there is a new column "myExtension" for the inheritance. It is is filled with the parent product ID for all variants, so everything looks good. But if I load a variant via DAL the extension is always "null". It is not taken from the parent product. What is missing in my code to achieve that?

source https://stackoverflow.com/questions/67820918/shopware-6-create-inherited-entity-extension-for-products