Django Shortcut nested foreign key

Question

Suppose I have the following in my models.py:

class Book:
    pass

class Part:
    book = models.ForeignKey(Book)

class Chapter:
    part = models.ForeignKey(Part)
    number = models.IntegerField()

I would like to do

book = Book.objects.get(id=someID)
chapters = Book.chapters.get(number=4)

What is a clean way to do so? I thought of a Manager on the book class but it doest not seem to work for this case.

Of course I could implement a method get_chapters on class book but i would like to avoid this.

any ideas ?

Answer 1

Using related_name arguments for FK fields, coupled with prefetch_related for a queryset, will allow you to fetch all info related to a book with minimal performance hit (each prefetch_related param calls a separate query).

class Book:
    pass

class Part:
    book = models.ForeignKey(Book, related_name="parts")

class Chapter:
    part = models.ForeignKey(Part, related_name="chapters")
    number = models.IntegerField()

# fetch a book and all related info w/ only 2 db hits
book = Book.objects.first().prefetch_related("parts","parts__chapters")
print(book.parts.all()) # returns all parts for book
for part in book.parts.all():
    print part.chapters.all()

You can do this in a template as well.

However, for the most performant solution, save a FK to book from chapter as well. This can be easily done by overriding the save method.

class Chapter:
    part = models.ForeignKey(Part, related_name="part_chapters")
    number = models.IntegerField()
    book = models.ForeignKey(Book, related_name="chapters", null=True, blank=True) # allow null/blank values; will be populated in save method
    def save(self, *args, **kwargs):
        self.book = self.part.book
        super(Chapter, self).save(*args, **kwargs)

>>> book = Book.objects.first().prefetch_related("parts","chapters")
>>> print(book.parts.all()) # returns all parts for book
>>> print(book.chapters.all())