CODEWITHSUNDEEP
djangofilter
praks5432
praks5432

Reputation: 7792

Django Filter Return Many Values

I'm new to django and I think this is a simple question -

I have an intermediate class which is coded as follows - 

class Link_Book_Course(models.Model):
    book = models.ForeignKey(Book)
    course = models.ForeignKey(Course)
    image = models.CharField(max_length = 200, null=True)
    rating = models.CharField(max_length = 200,null=True)
    def __unicode__(self):
        return self.title
    def save(self):
        self.date_created = datetime.now()
        super(Link_Book_Course,self).save()

I'm making this call as I'd like to have to have all of the authors of the books (Book is another model with author as a CharField)

    storeOfAuthorNames = Link_Book_Course.objects.filter(book__author)

However, it doesn't return a querySet of all of the authors, in fact, it throws an error.

I think it's because book__author has multiple values- how can I get all of them?

Thanks!

Upvotes: 1

Views: 1750

Answers (4)

nrabinowitz
nrabinowitz

Reputation: 55678

I don't think you're using the right queryset method. filter() filters by its arguments - so the expected usage is:

poe = Author.objects.get(name='Edgar Allen Poe')
course_books_by_poe = Link_Book_Course.objects.filter(book__author=poe)

It looks like you're trying to pull a list of the names all the authors of books used in a particular course (or maybe all courses?). Maybe you're looking for .values() or values_list()?

all_authors_in_courses = Link_Book_Course.objects.values_list(
        'book__author', flat=True
    ).distinct()

(Edit: Updated per @ftartaggia's suggestion)

Upvotes: 3

Tommaso Barbugli
Tommaso Barbugli

Reputation: 12031

As others already explained, the use of filter method is to get a subset of the whole set of objects and does not return instances of other models (no matter if related objects or so)

If you want to have Author models instances back from django ORM and you can use aggregation APIs then you might want to do something like this:

from django.db.models import Count
Author.objects.annotate(num_books=Count('book')).filter(num_books__gt=1)

Upvotes: 1

Vanessa MacDougal
Vanessa MacDougal

Reputation: 992

To start with, a filter statement filters on a field matching some pattern. So if Book has a simple ForeignKey to Author, you could have
storeOfAuthorNames = Link_Book_Course.objects.filter(book__author="Stephen King"), but not just
storeOfAuthorNames = Link_Book_Course.objects.filter(book__author).

Once you get past that, I am guessing Book has Author as a ManyToManyField, not a ForeignKey (because a book can have multiple authors, and an author can publish multiple books?) In that case, just filter(book__author="Stephen King") will still not be enough. Try Link_Book_Course.objects.filter(book_author__in=myBookObject.author.all())

Upvotes: 0

ftartaggia
ftartaggia

Reputation: 541

the filter method you are trying to use translates more or less into SQL like this:

SELECT * FROM Link_Book_Course INNER JOIN Book ON (...) WHERE Book.author = ;

So as you see your query has an incomplete where clause.

Anyway, it's not the query you are looking for.

What about something like (assuming author is a simple text field of Book and you want only authors of books referred from Link_Book_Course instances):

Book.objects.filter(pk__in=Link_Book_Course.objects.all().values_list("book", flat=True)).values_list("author", flat=True)

Upvotes: 0

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