未加星标

Django Tips #17 Using QuerySet Latest & Earliest Methods

字体大小 | |
[开发(python) 所属分类 开发(python) | 发布者 店小二04 | 时间 2016 | 作者 红领巾 ] 0人收藏点击收藏

Similar to the QuerySet methods first and last , the API also offer the earliest and latest methods. They are convenience methods which can be used to enhance the readability of the code.

The best way to use it is to define get_latest_by in the model’s Meta class:

class Post(models.Model): headline = models.CharField(max_length=150) author = models.ForeignKey(User, on_delete=models.CASCADE) publication_date = models.DateTimeField(blank=True, null=True) change_date = models.DateTimeField(blank=True, null=True) class Meta: get_latest_by = 'publication_date'

Then the usage is very straightforward:

latest_post = Post.objects.latest() earliest_post = Post.objects.earliest()

If you didn’t specify the get_latest_by property, or if you want to use a different field, you can pass it as a parameter on the fly:

latest_change = Post.objects.latest('change_date')

The earliest and latest methods will raise a DoesNotExist exception if there is no object with the given parameters, that is, the table is empty or it was filtered. It is slightly different from first and last , because it returns None if there is no matching object.

Another important thing to note is that the earliest and latest methods might return instances with null dates. But the thing is, the ordering behavior is not consistent between the different databases. So you might want to remove null dates, like so:

Post.objects.filter(change_date__isnull=False).latest('change_date')

Typically it is used with either DateField , DateTimeField or IntegerField . It will work with other field types. But you should avoid it, because it will be semantically wrong, and as those methods are available just for convinience and to enhance readability, in that case, using for something different than dates will cause more confusion.

本文开发(python)相关术语:python基础教程 python多线程 web开发工程师 软件开发工程师 软件开发流程

主题: Django
分页:12
转载请注明
本文标题:Django Tips #17 Using QuerySet Latest & Earliest Methods
本站链接:http://www.codesec.net/view/484065.html
分享请点击:


1.凡CodeSecTeam转载的文章,均出自其它媒体或其他官网介绍,目的在于传递更多的信息,并不代表本站赞同其观点和其真实性负责;
2.转载的文章仅代表原创作者观点,与本站无关。其原创性以及文中陈述文字和内容未经本站证实,本站对该文以及其中全部或者部分内容、文字的真实性、完整性、及时性,不作出任何保证或承若;
3.如本站转载稿涉及版权等问题,请作者及时联系本站,我们会及时处理。
登录后可拥有收藏文章、关注作者等权限...
技术大类 技术大类 | 开发(python) | 评论(0) | 阅读(35)