SilverStripe CMS: how to make a connection in form (select list) to another DataObject/Page in has_one fashion

Question

Let say I have a Event Page Object with should be connected to one Trainer. How Can I add to Event object a select list with All Trainers (Trainers are also a Page).

I Know hot to add a connection type by $has_one property, but I dont know how to popuate select with trainers objects.

Answer 1

$trainers = DataObject::get('Trainer');
$fields->addFieldToTab('Root.Content.Main', new DropdownField('TrainerID', 'Trainer', $trainers->map() );