Symfony form type entity data transformerm

I have form with field type entity on this field I have query_builder which return query and 'property' => 'name',. And my question what need to do in data transformer for change select name, need complicated with several filed, example - name_address_office. Using Symfony 2.8 I need dataTransformer approach

my form

class OutBoundInvoiceRowType extends AbstractType
{
/**
 * @var array
 */
private $vatClasses;

/**
 * @var Container
 */
private $container;

/**
 * @var EntityManager
 */
private $em;

/**
 * OutBoundInvoiceRowType constructor.
 * @param Container $container
 * @param $vatClasses
 */
public function __construct(
    Container $container,    
    $vatClasses
) {
    $this->container = $container;
    $this->vatClasses = $vatClasses;
    $this->em = $this->container->get('doctrine.orm.entity_manager');
}

/**
 * @param FormBuilderInterface $builder
 * @param array $options
 */
public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        ->add('location', 'entity', array(
            'class' => Location::class,
            'property' => 'name',
            'empty_value' => 'Choice Location',
            'query_builder' => self::getLocations(),
            'required' => false
        ))          
        ->add('vat', ChoiceType::class, [
            'choices' => ?,
            'required' => true,
        ])
    $builder->get('vat')
        ->addModelTransformer(new VatTransformer($this->container));
}

and my VatTransformer:

class VatTransformer implements DataTransformerInterface
{
    /**
     * @var Container
     */
    private $container;

    /**
     * @var EntityManager
     */
    private $em;

    /**
     * LocationTransformer constructor.
     * @param Container $container
     */
    public function __construct(Container $container)
    {
        $this->container = $container;
        $this->em = $this->container->get('doctrine.orm.entity_manager');
    }

    /**
     *
     * @param  Location|null $issue
     * @return string
     */
    public function transform($issue)
    {
        if (null === $issue) {
            return '';
        }
    }
}

in function transform $issue have null and when return '' nothing change in form, still have 'property' => 'name', on choice, What need to do in data transform name ?

this now I have

and this what I need

need name of several parts

UPDATE

Ok. I have choice field vat and I need build data in choice for vat field like - from some entity field, example entity Location (id, name)

How this realized with dataTransformer ?