django queryset API

Question

Suppose i have :

models.py:

class Books(models.Model):

    title = models.CharField(max_length = 200)
    author = models.CharField(max_length = 200)
    price = models.CharField(max_length = 200)

and in manage.py shell :

# hits the database
>>> book_one = Books.objects.get(id = 1)
# hits the database 
>>> foo = book_one.title
u'Foo book'

assuming on the code above, if i type book_one.author we're gonna hit the database again.

By using select_related() if i type book_one.title or book_one.author or book_one.price we're not gonna hit the database again, only when we instance it.

If i type books = Books.objects.all() in manage.py shell and let's say i want to store its specific value (let's say i only want to store title and author field) in an empty list using for loop like :

empty_list = []
for book in books:
    empty_list.append([book.title, book.author])
# only 1 data, so we're gonna loop only once

So the question is : based on that loop above, how many times we're gonna hit the database? Is it 2 times because we pass book.title and book.author or only once

you will not hit the database again for book_one.title or book_one.author — Konstant
– Konstant, Commented Sep 13, 2012 at 11:35
really? because the docs says it's gonna hit the database again docs.djangoproject.com/en/1.4/ref/models/querysets/… — Santana
– Santana, Commented Sep 13, 2012 at 11:37
what i meant was about your first statement.. # hits the database >>> book_one = Books.objects.get(id = 1) # hits the database -- [no it will not ] >>> book_one.title u'Foo book' — Konstant
– Konstant, Commented Sep 13, 2012 at 11:39

Ignacio Vazquez-Abrams · Accepted Answer · 2012-09-13 11:46:17Z

2

assuming on the code above, if i type book_one.author we're gonna hit the database again.

Incorrect. All simple fields in a model are pulled from the database unless .defer() or .only() are used.

By using select_related() if i type book_one.title or book_one.author or book_one.price we're not gonna hit the database again, only when we instance it.

select_related() only affects related fields, e.g. ForeignKey, OneToOneField, etc.

based on that loop above, how many times we're gonna hit the database?

Once. But consider using .values_list('title', 'author') instead.

answered Sep 13, 2012 at 11:46

Ignacio Vazquez-Abrams

804k160 gold badges1.4k silver badges1.4k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Konstant · Accepted Answer · 2012-09-13 11:46:12Z

0

If you have a class

class Books(models.Model):

    title = models.CharField(max_length = 200)
    author = models.CharField(max_length = 200)
    price = models.CharField(max_length = 200)

Now,

>>> book_one = Books.objects.get(id = 1) # hits the database
>>> foo = book_one.title # does not hit the database

if at all title was a foreign key to another table then, foo = book_one.title would have hit the database and using select_related would have saved you this database hit

answered Sep 13, 2012 at 11:46

Konstant

2,19916 silver badges32 bronze badges

Comments

Burhan Khalid · Accepted Answer · 2012-09-13 11:49:26Z

0

Your assumption is not correct. Querysets are lazy, and are only executed when evaluated.

f = Books.objects.get(pk=1) # one hit
f.title # no hit
f.price # no hit

answered Sep 13, 2012 at 11:49

Burhan Khalid

175k20 gold badges255 silver badges292 bronze badges

Collectives™ on Stack Overflow

django queryset API

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related