问题描述
我有这样一本书模型：
class Book(models.Model):
authors = models.ManyToManyField(Author, ...)
...
简而言之：
我想检索作者严格等同于一组给定作者的图书。我不知道是否有一个单一的查询，但是任何建议都将是有帮助的。
I'd like to retrieve the books whose authors are strictly equal to a given set of authors. I'm not sure if there is a single query that does it, but any suggestions will be helpful.
长：
这是我尝试的（没有运行得到AttributeError）
Here is what I tried, (that failed to run getting an AttributeError)
# A sample set of authors
target_authors = set((author_1, author_2))
# To reduce the search space,
# first retrieve those books with just 2 authors.
candidate_books = Book.objects.annotate(c=Count('authors')).filter(c=len(target_authors))
final_books = QuerySet()
for author in target_authors:
temp_books = candidate_books.filter(authors__in=[author])
final_books = final_books and temp_books
...这里是我所得到的：
... and here is what I got:
AttributeError: 'NoneType' object has no attribute '_meta'
一般来说，我应该如何查询一个模型，其约束是它的ManyToMany字段包含一组给定的对象在我的情况下？
In general, how should I query a model with the constraint that its ManyToMany field contains a set of given objects as in my case?
ps：我发现一些相关的SO问题，但无法得到一个明确的答案。任何好的指针也将有所帮助。谢谢。
ps: I found some relevant SO questions but couldn't get a clear answer. Any good pointer will be helpful as well. Thanks.
推荐答案
类似于@ goliney的方法，我找到了一个解决方案。但是，我认为效率可以提高。
Similar to @goliney's approach, I found a solution. However, I think the efficiency could be improved.
# A sample set of authors
target_authors = set((author_1, author_2))
# To reduce the search space, first retrieve those books with just 2 authors.
candidate_books = Book.objects.annotate(c=Count('authors')).filter(c=len(target_authors))
# In each iteration, we filter out those books which don't contain one of the
# required authors - the instance on the iteration.
for author in target_authors:
candidate_books = candidate_books.filter(authors=author)
final_books = candidate_books
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