Django: get related set from a related set of a mo

问题:

class Book(models.Model):
    # fields

class Chapter(models.Model):
     book = models.ForeignKey(Book)

class Page(models.Model):
     chapter = models.ForeignKey(Chapter)

I want all the pages of the book A, possibly without cycling every chapter to fetch the pages.

book = Book.objects.get(pk=1)
pages = book.chapter_set.page_set #?!?

回答1:

You can't do it that way. chapter_set is a query set, it doesn't have an attribute page_set.

Instead, turn it around:

Page.objects.filter(chapter__book=my_book)

回答2:

When you query cross models, double underscores may help

book = Book.objects.get(pk=1)
pages = Page.objects.filter(chapter__book=book)