尝试在Django中使用`comments` contrib实现联接时出现问题

Question

我有一个模型，评论是用django_comments控件来管理的：

class Fortune(models.Model):
    author = models.CharField(max_length=45, blank=False)
    title = models.CharField(max_length=200, blank=False)
    slug = models.SlugField(_('slug'), db_index=True, max_length=255, unique_for_date='pub_date')
    content = models.TextField(blank=False)
    pub_date = models.DateTimeField(_('published date'), db_index=True, default=datetime.now())
    votes = models.IntegerField(default=0)
    comments = generic.GenericRelation(
        Comment,
        content_type_field='content_type',
        object_id_field='object_pk'
    )

我想要检索Fortune对象，每个对象都有一个补充的nb_comments值，计算它们各自的注释数量；我尝试执行以下查询：

>>> Fortune.objects.annotate(nb_comments=models.Count('comments'))

在shell中：

>>> from django_fortunes.models import Fortune
>>> from django.db.models import Count
>>> Fortune.objects.annotate(nb_comments=Count('comments'))
[<Fortune: My first fortune, from NiKo>, <Fortune: Another One, from Dude>, <Fortune: A funny one, from NiKo>]
>>> from django.db import connection
>>> connection.queries.pop()
{'time': '0.000', 'sql': u'SELECT "django_fortunes_fortune"."id", "django_fortunes_fortune"."author", "django_fortunes_fortune"."title", "django_fortunes_fortune"."slug", "django_fortunes_fortune"."content", "django_fortunes_fortune"."pub_date", "django_fortunes_fortune"."votes", COUNT("django_comments"."id") AS "nb_comments" FROM "django_fortunes_fortune" LEFT OUTER JOIN "django_comments" ON ("django_fortunes_fortune"."id" = "django_comments"."object_pk") GROUP BY "django_fortunes_fortune"."id", "django_fortunes_fortune"."author", "django_fortunes_fortune"."title", "django_fortunes_fortune"."slug", "django_fortunes_fortune"."content", "django_fortunes_fortune"."pub_date", "django_fortunes_fortune"."votes" LIMIT 21'}

下面是格式正确的sql查询：

SELECT "django_fortunes_fortune"."id", 
       "django_fortunes_fortune"."author", 
       "django_fortunes_fortune"."title", 
       "django_fortunes_fortune"."slug", 
       "django_fortunes_fortune"."content", 
       "django_fortunes_fortune"."pub_date", 
       "django_fortunes_fortune"."votes", 
       COUNT("django_comments"."id") AS "nb_comments" 
FROM "django_fortunes_fortune" 
LEFT OUTER JOIN "django_comments" 
    ON ("django_fortunes_fortune"."id" = "django_comments"."object_pk") 
GROUP BY "django_fortunes_fortune"."id", 
         "django_fortunes_fortune"."author", 
         "django_fortunes_fortune"."title", 
         "django_fortunes_fortune"."slug", 
         "django_fortunes_fortune"."content", 
         "django_fortunes_fortune"."pub_date", 
         "django_fortunes_fortune"."votes" 
LIMIT 21

你能发现问题所在吗？Django不会将django_comments表与content_type数据(其中包含对fortune表的引用)连接在一起。

这是我希望能够使用ORM生成的查询：

SELECT "django_fortunes_fortune"."id", 
       "django_fortunes_fortune"."author", 
       "django_fortunes_fortune"."title", 
       COUNT("django_comments"."id") AS "nb_comments" 
FROM "django_fortunes_fortune" 
    LEFT OUTER JOIN "django_comments" 
        ON ("django_fortunes_fortune"."id" = "django_comments"."object_pk") 
    LEFT OUTER JOIN "django_content_type" 
        ON ("django_comments"."content_type_id" = "django_content_type"."id") 
GROUP BY "django_fortunes_fortune"."id", 
         "django_fortunes_fortune"."author", 
         "django_fortunes_fortune"."title", 
         "django_fortunes_fortune"."slug", 
         "django_fortunes_fortune"."content", 
         "django_fortunes_fortune"."pub_date", 
         "django_fortunes_fortune"."votes" 
LIMIT 21

但我不能做到这一点，所以Django老兵的帮助是非常感谢的:)

提示:我正在使用Django 1.2-DEV

提前感谢您的帮助。

Answer 1

http://charlesleifer.com/blog/generating-aggregate-data-across-generic-relations/ http://djangosnippets.org/snippets/2034/

更新：http://github.com/coleifer/django-generic-aggregation/

Answer 2

为什么要加入内容类型表？原始查询是错误的，但不是因为这个原因。对内容类型的引用是为了标识注释与哪个目标模型相关联--但是您已经知道这一点，因为您只选择了与Fortune相关联的注释。Django已经查询了ContentType模型来获取Fortune的值，所以应该只添加一个WHERE子句：

... WHERE django_comments_comment.content_type_id = xx

您也许可以通过添加与之等效的ORM来使其正常工作：

...filter(comment__content_type=ContentType.objects.get_for_model(Fortune))

尽管Django没有自动完成这一点，但这看起来确实是一个错误。

Answer 3

这是Django中的一个bug，请参阅ticket #10870。你至少要等到Django 1.3，或者自己修复它。