'Change the value of a column for a set of books. The ids parameter is a
list of book ids to change. The extra field must be None or a list the
same length as ids.'

def set_custom_bulk(self, ids, val, label=None, num=None, append=False, notify=True, extras=None):

    if ((extras is not None) and (len(extras) != len(ids))):
        raise ValueError('Length    of    ids    and    extras    is    not    the    same')
    ev = None
    for (idx, id) in enumerate(ids):
        if (extras is not None):
            ev = extras[idx]
        self._set_custom(id, val, label=label, num=num, append=append, notify=notify, extra=ev)
    self.dirtied(ids, commit=False)
    self.conn.commit()
