code: private function getCustomFieldValue(array $fields, $class, $slug)
      {
          return (isset($fields[$slug])) ? $this->provider
              ->getCustomFieldByType($this->getCustomField($class, $slug)->getType())
              ->deserialize($fields[$slug]) 
              : null;
      }
docs: Return the stored/raw value of a custom field.
  
  The method return null if the slug is not recorded.
  
  @param array $fields  the **raw** array, as stored in the db
  @param object|string $class
  @param string $slug
  @return mixed|null The value or null if the slug is not recorded