教义2个具有一对多关系的实体

entities\\Topic
    id    = 1            // integer
    title = \"example\"    // string
    comments             // entities\\Comment = oneToMany
        id   = 1                    // integer
        comment = \"first comment\"   // string
        topic                       // entities\\Topic = manyToOne
            id    = 1 // again..
            title = \"example\"

$this->em->createQueryBuilder()
         ->from(\'Entities\\Topic\', \'t\')
         ->select(\"t, c\")
         ->leftjoin(\"t.comments\", \'c\')
         ->where(\'t.id = :id\')
         ->setParameter(\'id\', 1)
         ->getQuery()->getSingleResult();