'Returns the latest object, according to the model\'s \'get_latest_by\'
option or optional given field_name.'

def latest(self, field_name=None):

    latest_by = (field_name or '_id')
    assert bool(latest_by), "latest()    requires    either    a    field_name    parameter    or    'get_latest_by'    in    the    model"
    obj = self._clone()
    obj._size = 1
    obj._clear_ordering()
    obj._ordering.append({latest_by: 'desc'})
    return obj.get()
