CODEWITHSUNDEEP
pythonsqldjangoperformance
Claudiu
Claudiu

Reputation: 229361

django: how many queries does this cause?

Say I have the following models:

class Baz(models.Model):
    winning = models.CharField(max_length=100)

class Bar(models.Model):
    baz = models.ForeignKey(Baz)

class Foo(models.Model):
    bar = models.ForeignKey(Bar)

Now I have an instance of Foo, foo. How many queries does the following line execute?

winning = foo.bar.baz.winning

Does it do one for each foreign-key ., or is Django smart enough to only do one query here?

Upvotes: 1

Views: 208

Answers (3)

errx
errx

Reputation: 1791

Here is the queries for winning = foo.bar.baz.winning statement: (I'am not counting Foo.objects.. statemnt)

(0.000) SELECT "foo_bar"."id", "foo_bar"."baz_id" FROM "foo_bar" WHERE "foo_bar"."id" = 1 ; args=(1,)
(0.000) SELECT "foo_baz"."id", "foo_baz"."winning" FROM "foo_baz" WHERE "foo_baz"."id" = 1 ; args=(1,)

And here is my tip: add logging for django.db.backends in your settings. https://docs.djangoproject.com/en/dev/topics/logging/#django-db-backends

And you will see all your queries in your console. Or install Django debug toolbar as Francis suggesting you.

Upvotes: 1

Yuji 'Tomita' Tomita
Yuji 'Tomita' Tomita

Reputation: 118458

That will require 3 queries.

One to get foo, bar, and baz.

Use select_related() to get them at once.

Foo.objects.get(id=1).select_related('bar__baz__winning')

Upvotes: 4

Francis Yaconiello
Francis Yaconiello

Reputation: 10919

There is a quick way to tell. Install Django Toolbar. Then check out the queries tab. It will give you all sorts of profiling info.

Though I believe +1 query per FK call

Upvotes: 2

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