Django：添加之前检查对象是否已存在 [英] Django: check whether an object already exists before adding

问题描述

这是一个非常简单的Django问题，尽管有很多猎头，但我在Django文档中找不到答案！

This is a pretty simple Django question, but I can't find the answer in the Django docs, despite lots of hunting!

如何检查是否存在对象已经存在，并且仅当它不已经存在时才添加它？

How do I check whether an object already exists, and only add it if it does not already exist?

这是代码-我不想添加follow_role数据库中两次（如果已经存在）。我如何首先检查？也许使用get（）-但是Django会抱怨get（）没有返回任何东西吗？

Here's the code - I don't want to add the follow_role twice in the database if it already exists. How do I check first? Use get() maybe - but then will Django complain if get() doesn't return anything?

current_user = request.user
follow_role = UserToUserRole(from_user=current_user, to_user=user, role='follow')
follow_role.save()

谢谢！

推荐答案

此惯用法有一个帮助函数，称为 get_or_create模型管理员：

There's a helper function for this idiom called 'get_or_create' on your model manager:

role, created = UserToUserRole.objects.get_or_create(
    from_user=current_user, to_user=user, role='follow')

它返回（model，bool）的元组，其中'model'是您感兴趣的对象，'bool'告诉您是否必须创建它。

It returns a tuple of (model, bool) where 'model' is the object you're interested in and 'bool' tells you whether it had to be created or not.

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