Doctrine: Update discriminator for SINGLE_TABLE Inheritance

Question

With these classes, how would you change a record for a "Person" to an "Employee".

/**
 * @Entity
 * @InheritanceType("SINGLE_TABLE")
 * @DiscriminatorColumn(name="discr", type="string")
 * @DiscriminatorMap({"person" = "Person", "employee" = "Employee"})
 */
class Person
{
    // ...
}

/**
 * @Entity
 */
class Employee extends Person
{
    // ...
}

I tried changing the value of the discriminator column but I can't access that. I also tried creating an 'Employee' instance and manually copy the data over but that doesn't work with auto-incrementing id's. It just gets added as a new record instead of updating the existing one.

Do I need to write a custom sql query or am I doing something else that is fundamentally wrong?

Answer 1

It is not a good sign when the type of an instance of an object needs to change over time. I'm not talking about downcasting/upcasting here, but about the need to change the real type of an object.

First of all, let me tell you why it is a bad idea:

1. A subclass might define more attributes and do some additionnal work in it's constructor. Should we run the new constructor again? What if it overwrites some of our old object's attributes?
2. What if you were working on an instance of that Person in some part of your code, and then it suddenly transforms into an Employee (which might have some redefined behavior you wouldn't expect)?!

That is part of the reason why most languages will not allow you to change the real class type of an object during execution (and memory, of course, but I don't want to get into details). Some let you do that (sometimes in twisted ways, e.g. the JVM), but it's really not good practice!

More often than not, the need to do so lies in bad object-oriented design decisions.

For those reasons, Doctrine will not allow you to change the type of your entity object. Of course, you could write plain SQL (at the end of this post - but please read through!) to do the change anyway, but here's two "clean" options I would suggest:

I realize you've already said the first option wasn't an option but I spent a while writing down this post so I feel like I should make it as complete as possible for future reference.

1. Whenever you need to "change the type" from Person to Employee, create a new instance of the Employee and copy the data you want to copy over from the old Person object to the Employee object. Don't forget to remove the old entity and to persist the new one.
2. Use composition instead of inheritance (see this wiki article for details and links to other articles). EDIT: For the hell of it, here's a part of a nice conversation with Erich Gamma about "Composition over Inheritance"!

See related discussions here and here.

---

Now, here is the plain SQL method I was talking about earlier - I hope you won't need to use it!

Make sure your query is sanitized (as the query will be executed without any verification).

$query = "UPDATE TABLE_NAME_HERE SET discr = 'employee' WHERE id = ".$entity->getId();
$entity_manager->getConnection()->exec( $query );

Here is the documentation and code for the exec method which is in the DBAL\Connection class (for your information):

/**
 * Execute an SQL statement and return the number of affected rows.
 *
 * @param string $statement
 * @return integer The number of affected rows.
 */
public function exec($statement)
{
    $this->connect();
    return $this->_conn->exec($statement);
}