Doctrine\ODM\MongoDB\DocumentManager::createDBRef PHP Method

    public function createDBRef($document, array $referenceMapping = null)
    {
        if (!is_object($document)) {
            throw new \InvalidArgumentException('Cannot create a DBRef, the document is not an object');
        }
        $class = $this->getClassMetadata(get_class($document));
        $id = $this->unitOfWork->getDocumentIdentifier($document);
        if ($id === null) {
            throw new \RuntimeException(sprintf('Cannot create a DBRef for class %s without an identifier. Have you forgotten to persist/merge the document first?', $class->name));
        }
        if ($referenceMapping['storeAs'] === ClassMetadataInfo::REFERENCE_STORE_AS_ID) {
            if ($class->inheritanceType === ClassMetadataInfo::INHERITANCE_TYPE_SINGLE_COLLECTION) {
                throw MappingException::simpleReferenceMustNotTargetDiscriminatedDocument($referenceMapping['targetDocument']);
            }
            return $class->getDatabaseIdentifierValue($id);
        }
        $dbRef = array('$ref' => $class->getCollection(), '$id' => $class->getDatabaseIdentifierValue($id));
        if ($referenceMapping['storeAs'] === ClassMetadataInfo::REFERENCE_STORE_AS_DB_REF_WITH_DB) {
            $dbRef['$db'] = $this->getDocumentDatabase($class->name)->getName();
        }
        /* If the class has a discriminator (field and value), use it. A child
         * class that is not defined in the discriminator map may only have a
         * discriminator field and no value, so default to the full class name.
         */
        if (isset($class->discriminatorField)) {
            $dbRef[$class->discriminatorField] = isset($class->discriminatorValue) ? $class->discriminatorValue : $class->name;
        }
        /* Add a discriminator value if the referenced document is not mapped
         * explicitly to a targetDocument class.
         */
        if ($referenceMapping !== null && !isset($referenceMapping['targetDocument'])) {
            $discriminatorField = $referenceMapping['discriminatorField'];
            $discriminatorValue = isset($referenceMapping['discriminatorMap']) ? array_search($class->name, $referenceMapping['discriminatorMap']) : $class->name;
            /* If the discriminator value was not found in the map, use the full
             * class name. In the future, it may be preferable to throw an
             * exception here (perhaps based on some strictness option).
             *
             * @see PersistenceBuilder::prepareEmbeddedDocumentValue()
             */
            if ($discriminatorValue === false) {
                $discriminatorValue = $class->name;
            }
            $dbRef[$discriminatorField] = $discriminatorValue;
        }
        return $dbRef;
    }

 /**
  * Checks that the current field includes a reference to the supplied document.
  */
 public function includesReferenceTo($document)
 {
     if ($this->currentField) {
         $this->query[$this->currentField][$this->cmd . 'elemMatch'] = $this->class ? $this->dm->createDBRef($document, $this->class->getFieldMapping($this->currentField)) : $this->dm->createDBRef($document);
     } else {
         $this->query[$this->cmd . 'elemMatch'] = $this->dm->createDBRef($document);
     }
     return $this;
 }