How to customize an entity property in Sylius?

I'm working on a Sylius application and want to modify a property of an entity.

To be more concrete: What I want to achieve, is to make the ProductVariant.onHand (or actually the corresponding column in the database) nullable.

The documentation of Sylius provides an auspicious article "Customizing Models". But it doesn't describe, how to change the definition of an existing property.

How to modify a property of a Sylius (Core) entity like ProductVariant.onHand?

What I tried so far: I extended the Sylius\Component\Core\Model\ProductVariant and added a Doctrine annotation to the onHand property:

/**
 * @ORM\Entity
 * @ORM\Table(name="sylius_product_variant")
 */
class ProductVariant extends BaseProductVariant
{
    ...
    /**
     * ...
     * @ORM\Column(type="integer", nullable=true)
     */
    protected $onHand = 0;
    ...
}

Well, extending the class was definitely a correct step. And it also worked correctly:

$ bin/console debug:container --parameter=sylius.model.product_variant.class
 ------------------------------------ ----------------------------------- 
  Parameter                            Value                              
 ------------------------------------ ----------------------------------- 
  sylius.model.product_variant.class   App\Entity\Product\ProductVariant  
 ------------------------------------ -----------------------------------

But the naïve adding of the property definition led to an error:

$ ./bin/console doctrine:schema:validate
  Property "onHand" in "App\Entity\Product\ProductVariant" was already declared, but it must be declared only once

Solution

Entity configs can be overridden by adding the AttributeOverrides annotation:

/**
 * @ORM\Entity
 * @ORM\Table(name="sylius_product_variant")
 * @ORM\AttributeOverrides({
 *     @ORM\AttributeOverride(
 *         name="onHand",
 *         column=@ORM\Column(
 *             name="on_hand",
 *             type="integer",
 *             nullable=true
 *         )
 *     )
 * })
 */
class ProductVariant extends BaseProductVariant
{
    ...
    /**
     * ...
     * no changes
     */
    protected $onHand;
    ...
}

Relevant Doctrine docu articles: