'Essentially, the opposite of defer. Only the fields passed into this
method and that are not already specified as deferred are loaded
immediately when the queryset is evaluated.'

def only(self, *fields):

    if (fields == (None,)):
        raise TypeError('Cannot    pass    None    as    an    argument    to    only().')
    clone = self._clone()
    clone.query.add_immediate_loading(fields)
    return clone
