Well I think it's important to check if it's namespace alias for sure, so we can have temporary mapping in $fqcn like:
```php
$fqcn = array(
    'AcmeBundle:User' => 'Acme\SomeBundle\Entity\User',
    'AcmeBundle:Group' => 'Acme\SomeBundle\Entity\Group',
);
```
so we won't have to get metadata each time 'AcmeBundle:User' occurs. In fact I could remake three lines below to form:
```php
$fqcn[$alias] = $em->getClassMetadata($alias)->getName();
```