Re: Confused about this aggregation example from the docs

'book' (the model name) is the default value of ForeignKey.related_name for the publisher field on Book.

class Book(models.Model):
    publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE)

https://docs.djangoproject.com/en/stable/ref/models/fields/#django.db.models.ForeignKey.related_name

On Saturday, June 25, 2016 at 10:11:44 AM UTC-4, Ankush Thakur wrote:

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

--
You received this message because you are subscribed to the Google Groups "Django users" group.
To unsubscribe from this group and stop receiving emails from it, send an email to django-users+unsubscribe@googlegroups.com.
To post to this group, send email to django-users@googlegroups.com.
Visit this group at https://groups.google.com/group/django-users.
To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/5ee2d7cb-4247-45e0-9bb0-e5cb8013b04c%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.