public function validateReferences()
    {
        foreach ($this->referenceMappings as $fieldName) {
            $mapping = $this->mappings[$fieldName];
            if (!empty($mapping['targetDocument']) && !ClassLoader::classExists($mapping['targetDocument'])) {
                throw MappingException::invalidTargetDocumentClass($mapping['targetDocument'], $this->name, $mapping['fieldName']);
            }
        }
    }