// getCMSFields() on Product.php
$fields->addFieldToTab("Root.Content.Main", new TreeMultiselectField("Parents", "Product Groups","SiteTree"));

/**
* Sets the ParentID for the component set
* which handles our "multiple parents"
*/
function setParentID($id) {   
   // if ParentID exists, delete it from the Product_Parents table
   if($parentToDelete = $this->getField('ParentID')){
      $this->Parents()->remove($parentToDelete);
   }
   
   // Add the new parent to the Product_Parents table   
   $parents = $this->Parents();
   $parents->add($id);
   
   // Set the ParentID on the SiteTree object for new and reorganise behaviour
   $this->setField('ParentID', $id);
}
   
/**
* Called when we try and set the Parents() component set
* by Tree Multiselect Field in the administration.
*/
function onChangeParents(&$items) {
   // This ensures this product can never be a parent of itself
   if($items){
      foreach($items as $k => $id){
         if($id == $this->ID)
            unset($items[$k]);
      }
   }   
   return true;
}

/**
* Return the classes to appear on this node in the CMS tree.
* We add 'manyparents' to indicate that this node may appear more than
* once in the tree.
*/
function CMSTreeClasses($controller) {
   return parent::CMSTreeClasses($controller) . ' manyparents';
}

/**
* If this object is reorganised in the CMS, we need to update the Parents field,
* otherwise the order will be messed up when we next press save
*/
function cmsCleanup_parentChanged() {
   $parents = $this->Parents();
   foreach($parents as $parent) $parentIDs[] = $parent->ID;
   $parentVal = implode(",", $parentIDs);
      
   return parent::cmsCleanup_parentChanged() . " $('Form_EditForm').elements.Parents.treeDropdownField.setValue('$parentVal');";
}