'Returns the amount of unread messages between two users
:param um_to_user:
A Django :class:`User` for who the messages are for.
:param um_from_user:
A Django :class:`User` from whom the messages originate from.
:return:
An integer with the amount of unread messages.'

def count_unread_messages_between(self, um_to_user, um_from_user):

    unread_total = self.filter(message__sender=um_from_user, user=um_to_user, read_at__isnull=True, deleted_at__isnull=True).count()
    return unread_total
