/**
* Get the key for a sort index on a relationship
*
* @param Relationship $relationship Relationship
* @param string $sort_field Property name on the inverse entity
* @param string $id Local ID
* @return string
*/
public function getSortIndexKey(Relationship $relationship, $sort_field, $id)
{
// srt:category-article:89726:title
return static::SORT_NAMESPACE . $this->delimiter . $relationship->getSourceTable() . '-' . $relationship->getTargetTable() . $this->delimiter . $id . $this->delimiter . $relationship->getName() . $this->delimiter . $sort_field;
}