Doctrine 2 - How to use discriminator column in where clause

It would look in query builder like this:

$class = 'Entity\File\Image';

$qb = $this->createQueryBuilder('f');
$qb->where($qb->expr()->isInstanceOf('f', $class));

Note: that you will not be able to set the class as a parameter because it will be escaped.

I think that you should use INSTANCE OF

for PHP 5.50 and above:

$this->createQueryBuilder('f')
        ->andWhere('f INSTANCE OF '.Image::class)