prefetch_related for multiple Levels

Question

If my Models look like:

class Publisher(models.Model):     pass  class Book(models.Model):     publisher = models.ForeignKey(Publisher)  class Page(models.Model):     book = models.ForeignKey(Book) 

and I would like to get the queryset for Publisher I do Publisher.object.all(). If then want to make sure to prefetch I can do:

Publisher.objects.all().prefetch_related('book_set')` 

My questions are:

1. Is there a way to do this prefetching using select_related or must I use prefetch_related?
2. Is there a way to prefetch the page_set? This does not work:

Publisher.objects.all().prefetch_related('book_set', 'book_set_page_set')

Answer 1

Since Django 1.7, instances of django.db.models.Prefetch class can be used as an argument of .prefetch_related. Prefetch object constructor has a queryset argument that allows to specify nested multiple levels prefetches like that:

Project.objects.filter(         is_main_section=True     ).select_related(         'project_group'     ).prefetch_related(         Prefetch(             'project_group__project_set',             queryset=Project.objects.prefetch_related(                 Prefetch(                     'projectmember_set',                     to_attr='projectmember_list'                 )             ),             to_attr='project_list'         )     ) 

It is stored into attributes with _list suffix because I use ListQuerySet to process prefetch results (filter / order).

Answer 2

1. No, you cannot use select_related for a reverse relation. select_related does a SQL join, so a single record in the main queryset needs to reference exactly one in the related table (ForeignKey or OneToOne fields). prefetch_related actually does a totally separate second query, caches the results, then "joins" it into the queryset in python. So it is needed for ManyToMany or reverse ForeignKey fields.

2. Have you tried two underscores to do the multi level prefetches? Like this: Publisher.objects.all().prefetch_related('book_set', 'book_set__page_set')