Use Doctrine to merge entity with different subclass

Question

I have a mapped superclass AbstractQuestion with single-table-inheritance.

/**
 * @ORM\Entity
 * @ORM\MappedSuperclass
 * @ORM\Table(name="Question")
 * @ORM\InheritanceType("SINGLE_TABLE")
 * @ORM\DiscriminatorColumn(name="dtype", type="string")
 * @ORM\DiscriminatorMap({
 *  "simple":   "SimpleQuestion",
 *  "dropdown": "DropdownQuestion"
 * })
 */
abstract class AbstractQuestion

SimpleQuestion and DropdownQuestion inherit from this superclass.

/**
 * Class SimpleQuestion.
 * @ORM\Entity()
 */
class SimpleQuestion extends AbstractQuestion

I want to modify an existing SimpleQuestion and make it a DropdownQuestion.

When saving a question, I deserialise and merge the question, which contains an ID and the 'dtype' and other properties.

$dquestion = $this->serial->fromJson($request->getContent(), AbstractQuestion::class);
$question = $this->em->merge($dquestion);
$this->em->flush();

So I submit something like:

{ id: 12, dtype: 'dropdown', 'text': 'What is my favourite animal?'}

After the deserialisation, $dquestion is a DropdownQuestion object as I desired, but after the merge $question is a SimpleQuestion object as it was in the database previously, so any unique properties of DropdownQuestion are lost and the question is saved as a SimpleQuestion. Is there any way to work around this?

Answer 1

You will first have to delete the existing record (SimpleQuestion) and then insert the new record (DropdownQuestion). Type casting is not supported in Doctrine 2.

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Note.
You can probably change the discriminator column with a pure SQL query, but this is absolutely not recommended and will for sure give you problems...

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