select_related multiple tables

Question

i have 3 models:

class book(models.Model):
    id = models.AutoField(primary_key=True)
    publisher= models.ForeignKey(Publisher, related_name='publisher')
    def __str__(self):
        return str(self.publisher)
    def __repr__(self):
        return str(self.publisher.id)
class reader(models.Model):
    publisher= models.ForeignKey(Publisher)
class publisher(models.Model):
    date = models.DateTimeField()

I'm trying to get all the readers who reads specific book, i dont have connection between reader and book, only via publisher table, i tried to get this information with: Book.object.select_related(publisher__readers__id='x') but I'm getting error. It is possible to get information from multiple tables like my model? thanks

If you want help tracking down an error, show the actual code and the error you're getting. The answer below shows a clearly better way to set up your code, but I also see some other issues. `Book.object` should raise an attribute error because it should be `Book.objects`. Also: `select_related` does not actually fetch anything, it just sets the Queryset up to do a prefetch when you actually execute the query. — RishiG, Jun 07 '18 at 15:21

score 2 · Answer 1 · answered Jun 07 '18 at 13:58

I'm feeling your code a bit weird, if you follow the Django Good Practices, related_name parameters and some sense of what you want to do, it should be something like:

class Book(models.Model):
    publisher = models.ForeignKey(Publisher, related_name='books')


class Reader(models.Model):
    publisher = models.ForeignKey(Publisher, related_name='readers')


class Publisher(models.Model):
    date = models.DateTimeField()

And then you have:

book = Book.objects.get(pk='<book_pk>')
readers = book.publisher.readers.all()
# or just
readers = Reader.objects.filter(publisher__books__pk='<book_pk>')

Your relation looks weird because you are creating a relation between between the Reader and the Publisher, instead of doing it with the Book itself. You should create a ManyToMany relation between Reader and Book, by doing that you have the scenario where: "A reader reads books, and more than one reader can read the same book.". Today you code means something like: "A read relates with a Publisher, which means that this Reader read all the Publisher Books", i don't see a lot of sense on it, sorry.

select_related multiple tables

1 Answers1