如何向django admin（顯示在模型儀表板右側的過濾器）添加自定義過濾器？我知道很容易包含一個基于該模型字段的過濾器，但是這樣的“計算”字段呢：class NewsItem(models.Model):    headline = models.CharField(max_length=4096, blank=False)    byline_1 = models.CharField(max_length=4096, blank=True)    dateline = models.DateTimeField(help_text=_("date/time that appears on article"))    body_copy = models.TextField(blank=False)    when_to_publish = models.DateTimeField(verbose_name="When to publish",  blank=True, null=True)    # HOW CAN I HAVE "is_live" as part of the admin filter?  It's a calculated state!!    def is_live(self):        if self.when_to_publish is not None:            if ( self.when_to_publish < datetime.now() ):                return """ <img alt="True" src="/media/img/admin/icon-yes.gif"/> """        else:            return """ <img alt="False" src="/media/img/admin/icon-no.gif"/> """          is_live.allow_tags = Trueclass NewsItemAdmin(admin.ModelAdmin):    form = NewsItemAdminForm    list_display = ('headline', 'id', 'is_live')    list_filter = ('is_live')  #  how can i make this work??