silverstripe-framework/model/fieldtypes/PrimaryKey.php

<?php
/**
 * A special type Int field used for primary keys.
 *
 * @todo Allow for custom limiting/filtering of scaffoldFormField dropdown
 *
 * @package framework
 * @subpackage model
 */
class PrimaryKey extends DBInt {
	/**
	 * @var DataObject
	 */
	protected $object;

	private static $default_search_filter_class = 'ExactMatchFilter';

	/**
	 * @param string $name
	 * @param DataOject $object The object that this is primary key for (should have a relation with $name)
	 */
	public function __construct($name = null, $object) {
		$this->object = $object;
		parent::__construct($name);
	}

	public function scaffoldFormField($title = null, $params = null) {
		$titleField = ($this->object->hasField('Title')) ? 'Title' : 'Name';
		$map = DataList::create(get_class($this->object))->map('ID', $titleField);
		$field = new DropdownField($this->name, $title, $map);
		$field->setEmptyString(' ');
		return $field;
	}
}