django – 渲染时捕获DoesNotExist：不存在照片匹配查询

photo = Photo.objects.get(original_image__exact=file_name) 
val = photo.admin_thumbnail.url

我收到此错误：

Caught DoesNotExist while rendering: Photo matching query does not exist.

这是我的班级：

class AdminImageWidget(forms.FileInput):
    """
    A ImageField Widget for admin that shows a thumbnail.
    """

    def __init__(self,attrs={},*args,**kwargs):
        super(AdminImageWidget,self).__init__(attrs)

    def render(self,name,value,attrs=None):
        output = []
        file_name = unicode(value)

        if file_name:
            photo = Photo.objects.get(original_image__exact=file_name) 
            val = photo.admin_thumbnail.url

            output.append(('<a target="_BLANK" href="%s">'
                           '<img src="%s" /></a> '
                           % (val,val)))
        output.append(super(AdminImageWidget,self).render(name,attrs))
        return mark_safe(u''.join(output))

但是,如果我在shell(python manage.py shell)中执行它,它的工作完美！

怪啊？