public function addParent(ItemAbstract $pItem)
    {
        if (! $pItem->getId()) {
            throw new Exception("addParent: Parent must exist in database");
        }

        $parentsValue = $this->getFieldValue($pItem->getIdField(), true);
        if (! $parentsValue) {
            $parents = array();
        } else {
            $parents = explode(',', $parentsValue);
        }

        $parents[] = $pItem->getId()->getOrig();
        $parents   = array_unique($parents);

        $this->_fields[$pItem->getIdField()] = implode(',', $parents);

        return $this;
    }