Symfony, can't chain getter methods

Symfony 6
Doctrine 3

Titelmeta Entity Class

<?php

namespace App\Entity;

use Doctrine\DBAL\Types\Types;
use Doctrine\ORM\Mapping as ORM;
use DateTime;

/**
 * Titelmeta
 *
  
  …

 * @ORM\Entity
 */
class Titelmeta
{
    …
	
    /**
     * @var \Isbn
     *
     * @ORM\Column(name="ISBN_mobi", type="bigint", nullable=false, options={"unsigned"=true})
     * @ORM\ManyToOne(targetEntity="Isbn")
     * @ORM\JoinColumns({
     *   @ORM\JoinColumn(name="ISBN_mobi", referencedColumnName="isbn")
     * })
     */
    private $isbnMobi;

	…

    public function getIsbnMobi(): ?Isbn
    {
        return $this->isbnMobi;
    }


}

Isbn Entity Class

<?php

namespace App\Entity;

use Doctrine\DBAL\Types\Types;
use Doctrine\ORM\Mapping as ORM;

/**
 * Isbn
 *
 * @ORM\Table(name="isbn", uniqueConstraints={@ORM\UniqueConstraint(name="isbn", columns={"isbn"})})
 * @ORM\Entity
 */
class Isbn
{
    /**
     * @var int
     *
     * @ORM\Column(name="isbn", type="bigint", nullable=false, options={"unsigned"=true})
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="IDENTITY")
     */
    private $isbn;

	…

    public function __toString()
    {
        return (string) $this->getIsbn();
    }

    public function getIsbn(): ?string
    {
        return $this->isbn;
    }

	…

}

I have an result object named $record

dump($record);

> App\Entity\Titelmeta {#901 ▼
-isbnMobi: "97812345678"
}

dump($record->getIsbnMobi()->getIsbn());

> App\Entity\Titelmeta::getIsbnMobi(): Return value must be of type
> ?App\Entity\Isbn, string returned

I don't get it. Can someone please help.

The isbnMobi should NOT be a number. In an ORM if you have relations between objects, it should always be null, an actual object, a proxy of an object or in case of multiple objects, a (proxy) collection. Since your isbnMobi is not an object but a number, this implies your ORM was misconfigured ...

The Titlemeta entity should NOT have @ORM\Column on the isbnMobi property. That information is handled via ManyToOne's JoinColumn. Remove it and you should be fine.

The Columns attributes can mostly be assigned to the JoinColumn instead. By default, the database column type must (automatically) match the referenced database column type, with nullable being the notable exception.

Also, the @var on that same property should be just Isbn not \Isbn, since the leading \ would mean it's in the root namespace.

If you're using php 8, use attributes instead of annotations and add the typehints to the property instead of @var (not technically binding).