tinternet.net/src/Repository/Blog/PostRepositoryQuery.php

<?php

namespace App\Repository\Blog;

use App\Entity\Blog\Category;
use App\Repository\RepositoryQuery;
use Knp\Component\Pager\PaginatorInterface;

/**
 * class PostRepositoryQuery.
 *
 * @author Simon Vieille <simon@deblan.fr>
 */
class PostRepositoryQuery extends RepositoryQuery
{
    public function __construct(PostRepository $repository, PaginatorInterface $paginator)
    {
        parent::__construct($repository, 'p', $paginator);
    }

    public function inCategory(Category $category)
    {
        $c = 'c'.mt_rand();

        $this
            ->innerJoin('p.categories', $c)
            ->andWhere($c.'.id = :category')
            ->setParameter(':category', $category->getId())
        ;

        return $this;
    }
}