Using Tags in an Extension
From Joomla! Documentation
Also bind() on component's table class on JTable should be overridden. Metadata should be json encoded before preStoreProcess.
public function bind($array, $ignore = '')
{
if (isset($array['metadata']) && is_array($array['metadata']))
{
$registry = new JRegistry;
$registry->loadArray($array['metadata']);
$array['metadata'] = (string) $registry;
}
return parent::bind($array, $ignore);
}