core: "No resource class found" if related entity is not explicitly in the file

Hello guys, firstly thanks for your amazing work.

I would like to report something that is not specified in the documentation and that could save much time to anyone experiencing the same issue in the future.

I have 3 entities : Ground, HeadOffice and Address. A ground has an embeded (doctrine’s annotation Embeded) relation with address, and headOffice has an embedded relation with address too.

I wanted to use api platform for ground and headoffice endpoints. Everything works fine for ground, I get objects like this :

{
    "entitled": "STADE JEAN SPAETY 1",
    "address": {
      "street": "ROUTE D ENSISHEIM HR 2/3",
      "postalCode": "68190",
      "city": "UNGERSHEIM",
      "country": "France"
    }
}

But for headOffice I got this error : No resource class found for object of type “AppBundle\Entity\Address”.

After some debugging hours, I found out that the problem occured in src/Serializer/AbstractItemNormalizer.php in the method getAttributeValue, where it tries to get propertyMetadata from attribute : in the headOffice case it fails (type in property metadata returned is null)

After comparing all annotations and methods in Ground.php and in HeadOffice.php, I found out that the only difference was in the property setter for address : in ground class, the setter was type hinted “Address” but not in headOffice class

Ground :

    /**
     * Set address.
     *
     * @param Address $address
     *
     * @return Ground
     */
    public function setAddress(Address $address)
    {
        $this->address = $address;

        return $this;
    }

HeadOffice :

    /**
     * Set address.
     *
     * @param Address $address
     *
     * @return HeadOffice
     */
    public function setAddress($address) // <-- Note the missing type hint
    {
        $this->address = $address;

        return $this;
    }

After adding the type hint in headOffice, the error disappeared and I successfully managed to get my head offices.

Conclusion : If you don’t explicitly specify the type hint in your class, the metadata factory fails to guess your property type. I thought it could guess it from namespace (they are all in AppBundle\Entity namespace after all) or from annotation on address property but apparently not.

Is it an issue ? Is it possible to add a warning on doc, especially here : https://api-platform.com/docs/core/serialization-groups-and-relations#normalization ? Is it possible to modify something to guess this part ?

Thanks for your answers. I’m not a specialist in serializer and I would be happy if you can explain to me (and others) if this is a good idea or not !