SilverStripe page/file/external URL field setup

LinkSwitcher.js

/**
 * File: LinkSwitcher.js
 */

(function($) {
  $.entwine(function($) {
    /**
     * Class: .cms-edit-form .field.switchable-controller
     */
    $('.cms-edit-form .field.switchable-controller').entwine({
      onmatch: function() {
        var id = this.find('input:checked').val();
        this.showField(id);
      },
      // Show the field with the given ID. Looks at siblings, so multiple
      // groups of switchable fields will need to be in different tabs,
      // or wrapped in CompositeFields
      showField: function(id) {
        var form = this.closest('form'),
          formId = form.attr('id'),
          switchableFields = this.siblings().filter('.field.switchable');

        switchableFields.hide();
        switchableFields.filter(function() {
          // Rough match - ID may be #FormName_FieldName or #FormName_FieldName_Holder
          return this.id.match(formId + '_' + id);
        }).show();
      }
    });

    /**
     * Input: .cms-edit-form .field.switchable-controller input
     */
    $('.cms-edit-form .field.switchable-controller input').entwine({
      onclick: function() {
        var id = this.val(),
          controller = this.parents('.field.switchable-controller');

        controller.showField(id);
        this._super();
      }
    });
  });
})(jQuery);

MyObjectWithLink.php

<?php

class MyObjectWithLink extends DataObject {

	private static $db = array(
		'Title' => 'Varchar',
		'URL' => 'Varchar',
		'LinkTarget' => 'Varchar'
	);

	private static $has_one = array(
		'PageLink' => 'SiteTree',
		'FileLink' => 'File'
	);

	/**
	 * {@inheritdoc}
	 */
	public function getCMSFields() {
		Requirements::javascript('mysite/javascript/LinkSwitcher.js');
		$fields = parent::getCMSFields();

		// Hide undesired fields
		$fields->removeByName('PageLinkID');
		$fields->removeByName('FileLinkID');
		$fields->removeByName('URL');
		$fields->removeByName('LinkTarget');

		$linkTypeField = OptionsetField::create(
			'LinkType',
			'',
			array(
				'PageLinkID' => 'Link to a page on this site',
				'FileLink' => 'Link to a file on this site',
				'URL' => 'Link to another website'
			),
			($this->URL ? 'URL' : ($this->FileLinkID ? 'FileLink' : 'PageLinkID'))
		)->addExtraClass('switchable-controller');

		$internalLinkField = TreeDropdownField::create('PageLinkID', 'Linked page', 'SiteTree')
			->addExtraClass('switchable');

		$fileLinkField = UploadField::create('FileLink', 'Linked file')
			->addExtraClass('switchable');

		$externalLinkField = TextField::create('URL', 'Linked page')
			->setDescription('Please include the "http://" prefix')
			->addExtraClass('switchable');

		$targetField = DropdownField::create(
			'LinkTarget',
			'Link target',
			array(
				'' => 'Open in the same window',
				'_blank' => 'Open in a new window'
			)
		);

		$fields->addFieldsToTab(
			'Root.Main',
			array(
				$linkTypeField,
				$internalLinkField,
				$fileLinkField,
				$externalLinkField,
				$targetField
			)
		);

		return $fields;
	}

	/**
	 * @return string
	 */
	public function getLink() {
		$page = $this->PageLink();
		if($page->exists()) {
			return $page->Link();
		}

		$file = $this->FileLink();
		if ($file->exists()) {
			return $file->Link();
		}

		return $this->URL;
	}

}

Page.ss

<% loop $List('MyObjectWithLink') %>
	<a href="{$Link}" target="{$LinkTarget}">{$Title}</a>
<% end_loop %>