It's not possible to order by relation properties
The following code (in a custom repository) won't work if "proposedSessionType" is a relation..
protected $defaultOrderings = array( 'proposedSessionType' => QueryInterface::ORDER_ASCENDING, );
Also it does not work to order by properties of a relation like:
protected $defaultOrderings = array( 'proposedSessionType.title' => QueryInterface::ORDER_ASCENDING, );