private function getCollectionItemType($className, $fieldName)
    {
        $serializerMetadata = $this->serializerMetadataFactory->getMetadataForClass($className);
        if ($serializerMetadata === null) {
            return null;
        }
        if (!isset($serializerMetadata->propertyMetadata[$fieldName])) {
            return null;
        }

        $type = $serializerMetadata->propertyMetadata[$fieldName]->type;
        return isset($type['name'], $type['params'][0]['name']) && $type['name'] === 'array' ?
            $type['params'][0]['name'] :
            null;
    }