CODEWITHSUNDEEP
djangodjango-models
Alejandro Veintimilla
Alejandro Veintimilla

Reputation: 11553

What happens when I change the default return of __unicode__ method to pass multiples foreignkeys

I'm wondering, in terms of database efficiency, if I edit the return __unicode__ method and do something like this:

class AnyModel(models.Model):
    a_field = models.ForeignKey(A)
    b_field = models.ForeignKey(B)

    def __unicode__(self):
        return "A - B: %s - %s " % (self.a_field.a2.a3, self.b_field.b2.b3)

So, the __unicode__ method will hit the foreign key's foreign key's foreign keys, get a string and then return it.

... Is it too expensive? What criteria should I follow? Is there any general advice I could use? Is it better not to write this kind of __unicode__ methods?

Upvotes: 1

Views: 48

Answers (1)

Paulo Pessoa
Paulo Pessoa

Reputation: 2569

This is what happening with this structure:

def __unicode__(self):
        return "A - B: %s - %s " % (self.a_field.a2.a3, self.b_field.b2.b3)

Hits : 

(0.000) QUERY = 'SELECT "core_a"."id", "core_a"."a2_id" FROM "core_a" WHERE "core_a"."id" = %s' - PARAMS = (2,); args=(2,)
(0.000) QUERY = 'SELECT "core_a2"."id", "core_a2"."a3_id" FROM "core_a2" WHERE "core_a2"."id" = %s' - PARAMS = (1,); args=(1,)
(0.012) QUERY = 'SELECT "core_a3"."id" FROM "core_a3" WHERE "core_a3"."id" = %s' - PARAMS = (1,); args=(1,)
(0.000) QUERY = 'SELECT "core_b"."id", "core_b"."b2_id" FROM "core_b" WHERE "core_b"."id" = %s' - PARAMS = (1,); args=(1,)
(0.000) QUERY = 'SELECT "core_b2"."id", "core_b2"."b3_id" FROM "core_b2" WHERE "core_b2"."id" = %s' - PARAMS = (1,); args=(1,)
(0.005) QUERY = 'SELECT "core_b3"."id" FROM "core_b3" WHERE "core_b3"."id" = %s' - PARAMS = (1,); args=(1,)

But, if you do this,:

def __unicode__(self):
        a_relation = AnyModel.objects.select_related('a_field__a2__a3').get(pk=self.pk)
        b_relation = AnyModel.objects.select_related('b_field__b2__b3').get(pk=self.pk)
        return "A - B: %s - %s " % (a_relation.a_field.a2.a3, b_relation.b_field.b2.b3)

When execute the line :

a_relation = AnyModel.objects.select_related('a_field__a2__a3').get(pk=self.pk)

1 hit :

(0.000) QUERY = 'SELECT "core_anymodel"."id", "core_anymodel"."a_field_id", "core_anymodel"."b_field_id", "core_a"."id", "core_a"."a2_id", "core_a2"."id", "core_a2"."a3_id", "core_a3"."id" FROM "core_anymodel" INNER JOIN "core_a" ON ( "core_anymodel"."a_field_id" = "core_a"."id" ) INNER JOIN "core_a2" ON ( "core_a"."a2_id" = "core_a2"."id" ) INNER JOIN "core_a3" ON ( "core_a2"."a3_id" = "core_a3"."id" ) WHERE "core_anymodel"."id" = %s' - PARAMS = (1,); args=(1,)

and When execute the second line:

b_relation = AnyModel.objects.select_related('b_field__b2__b3').get(pk=self.pk)

+1 Hit: 

(0.000) QUERY = 'SELECT "core_anymodel"."id", "core_anymodel"."a_field_id", "core_anymodel"."b_field_id", "core_b"."id", "core_b"."b2_id", "core_b2"."id", "core_b2"."b3_id", "core_b3"."id" FROM "core_anymodel" INNER JOIN "core_b" ON ( "core_anymodel"."b_field_id" = "core_b"."id" ) INNER JOIN "core_b2" ON ( "core_b"."b2_id" = "core_b2"."id" ) INNER JOIN "core_b3" ON ( "core_b2"."b3_id" = "core_b3"."id" ) WHERE "core_anymodel"."id" = %s' - PARAMS = (1,); args=(1,)

And when execute

return "A - B: %s - %s " % (self.a_field.a2.a3, self.b_field.b2.b3)

Doesn't hit the database.

. 1 - Edit

You can join this, to :

def __unicode__(self):
        relation = AnyModel.objects.select_related('a_field__a2__a3', 'b_field__b2__b3').get(pk=self.pk)
        return "A - B: %s - %s " % (relation.a_field.a2.a3, relation.b_field.b2.b3)

and will hit only once.

(0.000) QUERY = 'SELECT "core_anymodel"."id", "core_anymodel"."a_field_id", "core_anymodel"."b_field_id", "core_a"."id", "core_a"."a2_id", "core_a2"."id", "core_a2"."a3_id", "core_a3"."id", "core_b"."id", "core_b"."b2_id", "core_b2"."id", "core_b2"."b3_id", "core_b3"."id" FROM "core_anymodel" INNER JOIN "core_a" ON ( "core_anymodel"."a_field_id" = "core_a"."id" ) INNER JOIN "core_a2" ON ( "core_a"."a2_id" = "core_a2"."id" ) INNER JOIN "core_a3" ON ( "core_a2"."a3_id" = "core_a3"."id" ) INNER JOIN "core_b" ON ( "core_anymodel"."b_field_id" = "core_b"."id" ) INNER JOIN "core_b2" ON ( "core_b"."b2_id" = "core_b2"."id" ) INNER JOIN "core_b3" ON ( "core_b2"."b3_id" = "core_b3"."id" ) WHERE "core_anymodel"."id" = %s' - PARAMS = (1,); args=(1,)

Upvotes: 2

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