Django 当我将_unicode__）方法的默认返回值更改为传递多个外键时会发生什么

我想知道，在数据库效率方面，我是否编辑return

\uuuuuunicode\uuuuuu

方法并执行如下操作：

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)

因此，

\uuuuuuunicode\uuuuu

方法将命中外键的外键，获取一个字符串，然后返回它

---

太贵了吗？我应该遵循什么标准？有什么一般性的建议我可以使用吗？最好不要编写这种

\uuuuuunicode\uuuu

方法吗

这就是这种结构所发生的情况：

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

点击次数：

(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,)

但是，如果你这样做

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)

执行该行时：

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

1击：

(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,)

当执行第二行时：

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

+1击：

(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,)

什么时候执行

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

不会命中数据库。

。 1-编辑

您可以加入此，以：

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)

而且只会击中一次

(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,)

---

在这里，您可以看到有关select_的文档，感谢您的完整回答。我会接受的。还有一个问题，你认为只做一次连接和点击值得吗？我更喜欢只点击一次。