Clarify QuerySet.select_related() example regarding "hitting the database"

[Дилян Палаузов]

​https://docs.djangoproject.com/en/2.0/ref/models/querysets/#select-related says:

… then a call to Book.objects.select_related('author_ _hometown').get(id=4) will cache the related Person and the related City:

b = Book.objects.select_related('author_ _hometown').get(id=4)
p = b.author         # Doesn't hit the database.
c = p.hometown       # Doesn't hit the database.

b = Book.objects.get(id=4) # No select_related() in this example.
p = b.author         # Hits the database.
c = p.hometown       # Hits the database.

This leaves the impression, that the first snipped does not hit the database and the second snippet hits twice the database.

I propose:

• wring after b = Book.objects.select_related('author_ _hometown').get(id=4) the comment # Hits the database with INNER JOIN.
• changing b = Book.objects.get(id=4) # No select_related() in this example. to b = Book.objects.get(id=4) # No select_related() in this example, hits the database.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In e6f0e324:

Fixed #29081 -- Clarified comments in QuerySet.select_related() example.

[Tim Graham <timograham@…>]

In bbc4a5a:

[2.0.x] Fixed #29081 -- Clarified comments in QuerySet.select_related() example.

Backport of e6f0e324e28bc7d0c125415de871466aa1f4b983 from master