has_many.php

<?php

$relations = $this->stat('has_many');

$dropdowns = [];

foreach($relations as $name => $obj){
  $field = Dropdown::create($name)
    ->setTitle(Config::inst()->get($obj, 'singular_name'))
    ->setEmptyString('Select one')
    ->setSource($this->$name()->map('Title', 'Title'));
  array_push($dropdowns, $field);
}

$fields->addFieldsToTab('Root.Dropdowns', $dropdowns);