Django select_related does not work

django, django-orm, python

Solution

You're slightly misunderstanding how `selected_related` works. Referring to the django docs on `select_related`:

`select_related` returns a QuerySet that will “follow” foreign-key relationships, selecting additional related-object data when it executes its query. This is a performance booster which results in a single more complex query but means later use of foreign-key relationships won’t require database queries.

As you already determined, adding `select_related` causes django to select the related-object's data (`Publisher.id` & `Publisher.name` in this case). However, the `all()` method will still only return a `Book` QuerySet.

Where this is useful is when you access a `Book`'s `Publisher`, django will not need to query the database again for the `Publisher`:

# Hits the database.
# SELECT "Book"."id", "Book"."name", "Book"."publisher_id" ...
b = Book.objects.get(name='Twilight')

# Hits the database again to get the related Book object.
# SELECT "Publisher"."id", "Publisher"."name" ...
p = b.publisher

That's two database queries, whereas the `select_related` lookup is only one:

# Hits the database, but already includes Publisher data in the query
# SELECT "Book"."id", "Book"."name", "Book"."publisher_id", "Publisher"."id", "Publisher"."name" ...
b = Book.objects.select_related('publisher').get(name='Twilight')

# Doesn't hit the database, because b.publisher has been prepopulated
# in the previous query.
p = b.publisher

(example is a slight variation on the django docs example)

Problem

My django select_related works very weirdly ``` Models: class Publisher(models.Model): name = models.CharField(max_length=100) class Meta: app_label = 'models' db_table = 'Publisher' class Book(models.Model): name = models.CharField(max_length=100) publisher = models.OneToOneField(Publisher) class Meta: app_label = 'models' db_table = 'Book' ``` Output: ``` books = Book.objects.select_related('publisher').all() print books.query SELECT "Book"."id", "Book"."name", "Book"."publisher_id", "Publisher"."id", "Publisher"."name" FROM "Book" INNER JOIN "Publisher" ON ( "Book"."publisher_id" = "Publisher"."id" ) print books.values() [{'publisher_id': 1, u'id': 1, 'name': u'rest framework'}] ``` Django generates the correct query and data is retrieved when I execute it. But values do not contain Publisher

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