public function _getMongoCollection($writable = true)
{
if (is_null($this->getConfigAttribute('collection'))) {
require_once 'Shanty/Mongo/Exception.php';
throw new Shanty_Mongo_Exception('Can not fetch instance of MongoCollection. Document is not connected to a collection.');
}
return $this->_getMongoDb($writable)->selectCollection($this->getConfigAttribute('collection'));
}