SonataAdminBundle Exporter issue with mapped entities - symfony

Question

There is a standard feature in sonata-admin-bundle to export data using exporter; But how to make export current entity AND mapped ManyToOne entity with it?
Basically what I want, is to download exactly same data as defined in ListFields.
UPD: In docs, there is only todo
UPD2: I've found one solution, but I do not think it is the best one:
/**
* Add some fields from mapped entities; the simplest way;
* #return array
*/
public function getExportFields() {
$fieldsArray = $this->getModelManager()->getExportFields($this->getClass());
//here we add some magic :)
$fieldsArray[] = 'user.superData';
$fieldsArray[] = 'user.megaData';
return $fieldsArray;
}

Answer 1

I created own source iterator inherited from DoctrineORMQuerySourceIterator.
If value in method getValue is array or instance of Traversable i call method getValue recursive to get value for each "Many" entity:
protected function getValue($value)
{
//if value is array or collection, creates string
if (is_array($value) or $value instanceof \Traversable) {
$result = [];
foreach ($value as $item) {
$result[] = $this->getValue($item);
}
$value = implode(',', $result);
//formated datetime output
} elseif ($value instanceof \DateTime) {
$value = $this->dateFormater->format($value);
} elseif (is_object($value)) {
$value = (string) $value;
}
return $value;
}
In your admin class you must override method getDataSourceIterator to return your own iterator.
This
$this->getModelManager()->getExportFields($this->getClass());
returns all entity items. Better practice is to create explicit list of exported items in method getExportFields()
public function getExportFields()
{
return [
$this->getTranslator()->trans('item1_label_text') => 'entityItem1',
$this->getTranslator()->trans('item2_label_text') => 'entityItem2.subItem',
//subItem after dot is specific value from related entity
....
Key in array is used for export table headers (here is traslated).