How to filter a query using the max(date) value

How to filter a query using the max(date) value

I have this model:
class LogBook_audit(models.Model):
id = models.AutoField(primary_key=True)
logbook = models.ForeignKey(LogBook, verbose_name='Logbook
Control No.')
username = models.ForeignKey(User, null=True, blank=True,
verbose_name='Username')
status = models.CharField(null=False, blank=False,
max_length=40, choices=STATUS_TYPE, default=u'CHECKING REQUIREMENTS',
verbose_name='Status')
log_in = models.DateTimeField(default=lambda:datetime.now(),
verbose_name='Work start')
I'm trying to create a queryset to find latest user who log_in for such
specific logbook and status.
view.py
undolink(request, pk):
undo_rec = LogBook.objects.get(pk=pk)
latest_user = LogBook_audit.objects.filter(logbook_id=pk,
status=undo_rec.status).aggregate(Max('log_in'))
...
But latest_user is giving me a latest log_in format. I was thinking I can
used latest_user.username but failed. Is there a way to find that
username? Any guide is really appreciated.