Support for reverse field ordering

[dspacejs]

When using reverse ordering for a date field (such as -created_at) the following error is thrown:

AttributeError: 'Thread' object has no attribute '-created_at'

You can use reverse ordering like this using Django's Meta.ordering. But with Treebeard it doesn't work.

Here's my code as an example:

class Thread(MP_Node):
    node_order_by = ['-created_at']

    user = models.ForeignKey('User', on_delete=models.PROTECT, db_index=True)
    message = models.CharField(max_length=200)
    created_at = models.DateTimeField(default=timezone.now)

Is there a way to do this with Treebeard? My nodes need to be ordered by most recent.

Cheers!