Django - 限制查询结果 [英] Django - limiting query results

问题描述

我想要使用模型的最后10个实例，并具有以下代码：

I want to take the last 10 instances of a model and have this code:

 Model.objects.all().order_by('-id')[:10]

，然后只需要10个最后一个？
有没有更有效的方法？

Is it true that firstly pick up all instances, and then take only 10 last ones? Is there any more effective method?

推荐答案

Django查询器是懒惰的。这意味着只有当您特别要求结果时，查询才会触发数据库。

Django querysets are lazy. That means a query will hit the database only when you specifically ask for the result.

所以直到打印或实际使用查询的结果，您可以进一步过滤，无需数据库访问。

So until you print or actually use the result of a query you can filter further with no database access.

如下所示，您的代码只执行一个sql查询以仅获取最后10个项目。

As you can see below your code only executes one sql query to fetch only the last 10 items.

In [19]: import logging                                 
In [20]: l = logging.getLogger('django.db.backends')    
In [21]: l.setLevel(logging.DEBUG)                      
In [22]: l.addHandler(logging.StreamHandler())      
In [23]: User.objects.all().order_by('-id')[:10]          
(0.000) SELECT "auth_user"."id", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."password", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."is_superuser", "auth_user"."last_login", "auth_user"."date_joined" FROM "auth_user" ORDER BY "auth_user"."id" DESC LIMIT 10; args=()
Out[23]: [<User: hamdi>]

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