I've found I can merge dataobjects just fine, but I need to subtract Dataobjects too. It says the method subtract does not exist.

Is there a way to subtract $alreadypicked ?

$enhancements = new DataObjectSet();
$alreadypicked = DataObject::get('Enhancement', 'ProjectsID != 0');
$enhancements->merge(DataObject::get('Enhancement', 'StatusLevelsID = 8')); // In Progress
$enhancements->subtract($alreadypicked);
$enhancements->merge(DataObject::get('Enhancement', "ProjectsID = {$this->ID}"));

Thanks,
Brian

how about...

$enhancements = new DataObjectSet(); 
$alreadypicked = DataObject::get('Enhancement', 'ProjectsID != 0'); 
$enhancements->merge(DataObject::get('Enhancement', 'StatusLevelsID = 8')); // In Progress 
if ($alreadypicked) foreach ($alreadypicked as $item) {
   $enhancements->remove($item); 
}

but having looked again - what you are doing is crazy... why not just use a bit more SQL...

$enhancements = DataObject::get('Enhancement', 'StatusLevelsID = 8 AND ProjectsID != 0'); 

http://api.silverstripe.org/2.4/sapphire/model/DataObjectSet.html#methodremove

Thanks!!! Your second option was what I was originally looking for. I entered it incorrectly as:

$enhancements->merge(DataObject::get('Enhancement', 'StatusLevelsID = 8' AND 'ProjectsID != 0'))

Which gave me all StatusLevelsID of 8 and all ProjectsID of 0.

I was also wanting to know how to subtract (remove) an item too, so thanks again.

Brian