silverstripe-framework/model/fieldtypes/PolymorphicForeignKey.php

<?php

/**
 * A special ForeignKey class that handles relations with arbitrary class types
 *
 * @package framework
 * @subpackage model
 */
class PolymorphicForeignKey extends CompositeDBField {

	private static $composite_db = array(
		'ID' => 'Int',
		'Class' => 'DBClassName("DataObject")'
	);

	public function scaffoldFormField($title = null, $params = null) {
		// Opt-out of form field generation - Scaffolding should be performed on
		// the has_many end, or set programatically.
		// @todo - Investigate suitable FormField
		return null;
	}

	/**
	 * Get the value of the "Class" this key points to
	 *
	 * @return string Name of a subclass of DataObject
	 */
	public function getClassValue() {
		return $this->getField('Class');
	}

	/**
	 * Set the value of the "Class" this key points to
	 *
	 * @param string $value Name of a subclass of DataObject
	 * @param boolean $markChanged Mark this field as changed?
	 */
	public function setClassValue($value, $markChanged = true) {
		$this->setField('Class', $value, $markChanged);
	}

	/**
	 * Gets the value of the "ID" this key points to
	 *
	 * @return integer
	 */
	public function getIDValue() {
		return $this->getField('ID');
	}

	/**
	 * Sets the value of the "ID" this key points to
	 *
	 * @param integer $value
	 * @param boolean $markChanged Mark this field as changed?
	 */
	public function setIDValue($value, $markChanged = true) {
		$this->setField('ID', $value, $markChanged);
	}

	public function setValue($value, $record = null, $markChanged = true) {
		// Map dataobject value to array
		if($value instanceof DataObject) {
			$value = array(
				'ID' => $value->ID,
				'Class' => $value->class
			);
		}
		
		parent::setValue($value, $record, $markChanged);
	}

	public function getValue() {
		$id = $this->getIDValue();
		$class = $this->getClassValue();
		if($id && $class && is_subclass_of($class, 'DataObject')) {
			return DataObject::get_by_id($class, $id);
		}
	}
}