Question

I have such a Book model:

class Book(models.Model):
    authors = models.ManyToManyField(Author, ...)
    ...

In short:

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.

In long:

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

… and here is what I got:

AttributeError: 'NoneType' object has no attribute '_meta'

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: I found some relevant SO questions but couldn’t get a clear answer. Any good pointer will be helpful as well. Thanks.

Answer 1

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