public function compare($stack,$cpm_key='_id') {
		if ($stack instanceof CortexCollection)
			$stack = $stack->getAll($cpm_key,true);
		$keys = $this->getAll($cpm_key,true);
		$out = [];
		$new = array_diff($stack,$keys);
		$old = array_diff($keys,$stack);
		if ($new)
			$out['new'] = $new;
		if ($old)
			$out['old'] = $old;
		return $out;
	}