尝试在 Django 中使用“comments”contrib 实现连接时出现问题

Question

我有这个模型，cmets 由django_comments contrib 管理：

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'
    )

我想检索 Fortune 对象，并为每个对象添加一个补充 nb_comments 值，计算它们各自的 cmets 数量；我试试这个查询：

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

从外壳：

>>> 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'}

以下是格式正确的 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

你能发现问题吗？ Django 不会将django_comments 表与content_type 数据（其中包含对fortune 之一的引用）连接起来。

这是我希望能够使用 ORM 生成的那种查询：

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

但我做不到，因此非常感谢 Django 退伍军人的帮助 :)

提示：我使用的是 Django 1.2-DEV

提前感谢您的帮助。

Answer 1

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识别与 cmets 关联的目标模型 - 但您已经知道这一点，因为您只选择与 Fortune 关联的 cmets。 Django 已经查询了ContentType 模型以获取Fortune 的值，因此只需添加一个WHERE 子句：

... WHERE django_comments_comment.content_type_id = xx

您可以通过添加等效的 ORM 来使其正常工作：

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

虽然 Django 不会自动执行此操作似乎是一个错误。

Answer 2

这是 Django 中的一个错误，请参阅票证 #10870。您至少必须等待 Django 1.3 或自己修复它。

Answer 3

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

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