Confused about this aggregation example from the docs

On this page https://docs.djangoproject.com/en/1.9/topics/db/aggregation/ we have the following example of aggregation:

# All the following queries involve traversing the Book<->Publisher

# foreign key relationship backwards.

# Each publisher, each with a count of books as a "num_books" attribute.

>>> from django.db.models import Count

>>> pubs = Publisher.objects.annotate(num_books=Count('book'))

>>> pubs

[<Publisher BaloneyPress>, <Publisher SalamiPress>, ...]

>>> pubs[0].num_books

73

​The models used in this are as follows: 

from django.db import models

class Author(models.Model):

    name = models.CharField(max_length=100)

    age = models.IntegerField()

class Publisher(models.Model):

    name = models.CharField(max_length=300)

    num_awards = models.IntegerField()

class Book(models.Model):

    name = models.CharField(max_length=300)

    pages = models.IntegerField()

    price = models.DecimalField(max_digits=10, decimal_places=2)

    rating = models.FloatField()

    authors = models.ManyToManyField(Author)

    publisher = models.ForeignKey(Publisher)

    pubdate = models.DateField()

class Store(models.Model):

    name = models.CharField(max_length=300)

    books = models.ManyToManyField(Book)

    registered_users = models.PositiveIntegerField()

My question is: How come something like "Publisher.objects.annotate(num_books=Count('book'))" work? The name "book" is not defined as a reverse relationship (I think it should be accessible by "book_set"). 

How is this traversal working?

​

Regards,

Ankush Thakur

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