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Doctrine - Get row or rows from many results based on column value
$em = $this->getDoctrine()->getManager();

$query = $em->createQuery(
   'SELECT u, p FROM MYUserBundle:User u
   JOIN u.post p'
);

foreach($query->getResult() as $user){

   //a bunch of posts related to this user
   $posts = $user->getPosts();
}

$posts = $user->getPosts();
$post = $posts->findBySlug('my_slug');

//or something along those lines...

public function getPostsBySlug( $slug )
{
  return $this->posts->filter( function( Post $post ) use ( $slug )
  {
    return $post->getSlug() == $slug;
  } );
}

$posts = $user->getPostsBySlug( 'my_slug' );

$slug = 'my_slug';
array_filter($user->getPosts()->all(), function ($post) use ($slug) {
    return $post->getSlug() == $slug;
})

namespace AcmeThingBundleRepository;

use DoctrineORMEntityRepository;

class PostRepository extends EntityRepository
{
    public function getBySlug($slug)
    {
        /// your custom query here

        return $q->getQuery()->getResult();
    }
}