Django在预取过滤相关模型上进行注释

Question

我有一个 User 模型，每个 User 可以通过这个关系有很多 Post:

class Post(models.Model):
    ...
    user = models.ForeignKey('User', related_name='posts')

，并且每个帖子都有一个like_count(整数字段)，我想让用户(沿着他们的帖子)拥有超过 n 个帖子，并且被喜欢的次数超过 m 次，我这样做:

User.objects.prefetch_related(Prefetch('posts', queryset=Post.objects.filter(like_count__gte=m), to_attr='top_posts')).annotate(top_post_count=Count('top_posts')).filter(top_post_count__gte=n)

但是，我得到:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/query.py", line 912, in annotate
    clone.query.add_annotation(annotation, alias, is_summary=False)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/sql/query.py", line 971, in add_annotation
    summarize=is_summary)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/aggregates.py", line 19, in resolve_expression
    c = super(Aggregate, self).resolve_expression(query, allow_joins, reuse, summarize)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/expressions.py", line 513, in resolve_expression
    c.source_expressions[pos] = arg.resolve_expression(query, allow_joins, reuse, summarize, for_save)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/expressions.py", line 463, in resolve_expression
    return query.resolve_ref(self.name, allow_joins, reuse, summarize)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/sql/query.py", line 1462, in resolve_ref
    self.get_initial_alias(), reuse)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/sql/query.py", line 1402, in setup_joins
    names, opts, allow_many, fail_on_missing=True)
  File "/usr/local/lib/python3.4/dist-packages/django/db/models/sql/query.py", line 1327, in names_to_path
    "Choices are: %s" % (name, ", ".join(available)))
django.core.exceptions.FieldError: Cannot resolve keyword 'top_posts' into field. Choices are: ...

Answer 1

docs解释它被分配给一个属性，根据 to_attr 命名参数，在 QuerySet 项上。

我不认为这意味着它可以作为模型字段访问，随后的注释可以链接到该模型字段。

您可能需要将其分为两个步骤。

编辑:该问题评论说可以链接过滤器，但不能链接注释。