Doctrine\ODM\MongoDB\Aggregation\Stage\Lookup::from PHP Method

    public function from($from)
    {
        // $from can either be
        // a) a field name indicating a reference to a different document. Currently, only REFERENCE_STORE_AS_ID is supported
        // b) a Class name
        // c) a collection name
        // In cases b) and c) the local and foreign fields need to be filled
        if ($this->class->hasReference($from)) {
            return $this->fromReference($from);
        }
        // Check if mapped class with given name exists
        try {
            $targetMapping = $this->dm->getClassMetadata($from);
            return parent::from($targetMapping->getCollection());
        } catch (BaseMappingException $e) {
            return parent::from($from);
        }
    }