SilverStripe 3.x GridField with Orderable/Sortable ManyMany List

OrderableManyManyGridField.php

<?php

class Page extends SiteTree {
	private static $many_many = array(
		'RelatedPages' => 'Page',
	);
	// be careful to name the many_many_extraField something that does not conflict with existing DB fields. 
	// eg: 'Sort' already exists on class SiteTree, so using that will get you into trouble. 
	private static $many_many_extraFields = array(
		'RelatedPages' => array(
			'SortOnPage' => 'Int',
		),
	);

	public function getCMSFields() {
		$return = parent::getCMSFields();
		$return->addFieldToTab('Root', Tab::create('RelatedPages', $this->fieldLabel('RelatedPagesTab')));
		$return->addFieldToTab(
			'Root.RelatedPages',
			GridField::create(
				'RelatedPages',
				$this->fieldLabel('RelatedPages'),
				$this->getManyManyComponents('RelatedPages'),
				GridFieldConfig_RelationEditor::create()
					->removeComponentsByType('GridFieldAddNewButton')
					->addComponent(new GridFieldOrderableRows('SortOnPage'))
			)
		);
		return $return;
	}
	public function fieldLabels($includerelations = true) {
		$return = parent::fieldLabels($includerelations);
		$return['RelatedPagesTab'] = _t('Page.RelatedPagesTab', 'Related Pages');
		$return['RelatedPages'] = _t('Page.RelatedPages', 'Related Pages');
		return $return;
	}
	public function RelatedPages() {
		return $this->isInDB() ? $this->getManyManyComponents('RelatedPages')->sort('SortOnPage', 'ASC') : false;
	}
}