getReference() of doctrine Entity Manager

问题:

Situation:

I want to use the getReference() function of doctrine2 Entity Manager. However, in a situation where I ask for an object that has been deleted from the database, I obtain a proxy if I ask for that same object more than once.

An example:

//A random article object...that has been deleted from the database
$articleClass = 'Acme\ArticleBundle\Entity\Article';
$articleIdentifiers = array('id'=>1);
$i = 0;

//We ask for its reference twice
do{
    try {
        echo "a";
        $subject = $this->em->getReference(
            $subjectClass,
            $subjectIdentifiers
        );
       //call this object now
       var_dump($subject);

    } catch (\Exception $e) {
        echo "b";
    }
    $i++;
} while ($i <2);

The output:

a
b
a
object(Proxies\__CG__\Acme\ArticleBundle\Entity\Article)

How can I get a proxy for an object that doesn't even exist in the database? If I comment this line, the entityManager does not manage the object and I obtain the output abab, which to me makes more sense as I don't want to get a proxy object that does not exist in the database. For info, the proxy object returned has all its properties null. I therefore obtain a proxy for an object that does not exist in the database. So, if I ask for this object I get a "Not found Entity" exception.

The challenge

Can anyone make any sense of this? Is there a way to rely on getReference() to tell us whether this object really does exist in the database?

回答1:

There is no way to make getReference() check the database for the existence of the referenced object.

Actually, this is what getReference() and the proxies returned by it are all about: Creating placeholder objects (proxies) without going to the database. And you would rarely want to do that explicitly. Normally Doctrine does this internally when hydrating entities to create lazy loading placeholders for related entities based on foreign key values.

Why don't you just call find() on the Entity Manager? Are you aware that the EM will not query the DB more than once for the same object as long as you look it up by ID? Doctrine keeps track of already hydrated objects in the Unit Of Work and returns references to the existing objects in subsequent find() calls.

回答2:

If you use getReference to get an object, but then call a method on it such as getName then Doctrine will fetch the entity from the DB. It has no other way to find out the property (getName).

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Concerning your challenge:

EntityManager->contains($entity) would be the preferred way to check if the entity from doctrine is in the entity manager