Володя

$modx->exec("DELETE FROM {$this->modx->getTableName('msCategoryMember')} WHERE `product_id` = {$pid} AND `category_id` = {$cid};");

[size] => array(
	0 => 'двухспальный',
	1 => 'трехспальный',
)

$pid = $resourse->get('id'); // id товара
$cid = 55; // id категории
$res = $modx->getObject('msCategoryMember', array('category_id' => $cid, 'product_id' => $pid));
if (!$res) {
	$res = $modx->newObject('msCategoryMember');
	$res->set('product_id', $pid);
	$res->set('category_id', $cid);
	$res->save();
}

$key = $_REQUEST['fastrouter']['key'];
if ($modx->getCount('modContext', $key)) {
    $modx->switchContext($key);  
}

/{key}/item/{item_id:[0-9]+}/

[[!wiki-router?
 &context=`[[+vp.key]]`
 &item_id=`[[+vp.item_id]]`
]]

add($id, $count = 1, $options = array());