gistfile1.php

<?php
class Page extends SiteTree{
  private static $many_many =array(
    "PageSections"  =>  "PageSection"
  );
  private static $many_many_extraFields = array(
    'PageSections'=>array(
      'SortOrder' =>'Int'
    )
  );

  function getCMSFields() {
  	$f = parent::getCMSFields();
  	$gridconfig = GridFieldConfig_RelationEditor::create();
  	$gridconfig->addComponent(new GridFieldSortableRows('SortOrder'));
  	
    $f->addFieldToTab(
      'Root.Main', 
      $grid = new GridField('PageSection', 'Page Sections', $this->PageSections(), $gridconfig )
    );
    
    $f->removeByName("Metadata");
    return $f;
  }
  
  public function PageSections() {
    return $this->getManyManyComponents('PageSections')->sort('SortOrder');
  }
}
class Page_Controller extends ContentController{

}