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

    private function fromReference($fieldName)
    {
        if (!$this->class->hasReference($fieldName)) {
            MappingException::referenceMappingNotFound($this->class->name, $fieldName);
        }
        $referenceMapping = $this->class->getFieldMapping($fieldName);
        $targetMapping = $this->dm->getClassMetadata($referenceMapping['targetDocument']);
        parent::from($targetMapping->getCollection());
        if ($referenceMapping['isOwningSide']) {
            if ($referenceMapping['storeAs'] !== ClassMetadataInfo::REFERENCE_STORE_AS_ID) {
                throw MappingException::cannotLookupNonIdReference($this->class->name, $fieldName);
            }
            $this->foreignField('_id')->localField($referenceMapping['name']);
        } else {
            if (isset($referenceMapping['repositoryMethod'])) {
                throw MappingException::repositoryMethodLookupNotAllowed($this->class->name, $fieldName);
            }
            $mappedByMapping = $targetMapping->getFieldMapping($referenceMapping['mappedBy']);
            if ($mappedByMapping['storeAs'] !== ClassMetadataInfo::REFERENCE_STORE_AS_ID) {
                throw MappingException::cannotLookupNonIdReference($this->class->name, $fieldName);
            }
            $this->localField('_id')->foreignField($mappedByMapping['name']);
        }
        return $this;
    }