public function getCustomField($class, $slug = null)
    {
        if (!$slug) {
            throw CustomFieldsHelperException::slugIsMissing();
        }
        
        $resolveClass = (is_string($class)) ? $class : get_class($class);
        if (!$this->cache[$resolveClass][$slug]) {
            $this->_cacheCustomFields($resolveClass);
        }
        
        return $this->cache[$resolveClass][$slug];
    }