从 Django 中的查询集中获取第一个对象的最快方法? [英] Fastest way to get the first object from a queryset in django?

问题描述

我经常发现自己想要从 Django 的查询集中获取第一个对象，或者如果没有则返回 None.有很多方法可以做到这一点，这些方法都有效.但我想知道哪个性能最好.

Often I find myself wanting to get the first object from a queryset in Django, or return None if there aren't any. There are lots of ways to do this which all work. But I'm wondering which is the most performant.

qs = MyModel.objects.filter(blah = blah)
if qs.count() > 0:
    return qs[0]
else:
    return None

这会导致两次数据库调用吗?那看起来很浪费.这是更快吗?

Does this result in two database calls? That seems wasteful. Is this any faster?

qs = MyModel.objects.filter(blah = blah)
if len(qs) > 0:
    return qs[0]
else:
    return None

另一种选择是:

qs = MyModel.objects.filter(blah = blah)
try:
    return qs[0]
except IndexError:
    return None

这会生成单个数据库调用，这很好.但是很多时候需要创建一个异常对象，当你真正需要的只是一个微不足道的 if 测试时，这是一个非常占用内存的事情.

This generates a single database call, which is good. But requires creating an exception object a lot of the time, which is a very memory-intensive thing to do when all you really need is a trivial if-test.

如何仅通过一次数据库调用就可以做到这一点，并且不使用异常对象搅动内存?

How can I do this with just a single database call and without churning memory with exception objects?

推荐答案

使用方便的方法 .first() 和 .last():

Use the convenience methods .first() and .last():

MyModel.objects.filter(blah=blah).first()

如果查询集没有返回任何对象，它们都会吞下结果异常并返回None.

They both swallow the resulting exception and return None if the queryset returns no objects.

这些是在 Django 1.6 中添加的，它是 发布于2013 年 11 月.

These were added in Django 1.6, which was released in Nov 2013.

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