在Django中仅允许一个模型实例 [英] Allow only one instance of a model in Django

问题描述

我想使用数据库模型来控制我的项目的一些配置设置。例如：

I would like to control some configuration settings for my project using a database model. For example:

class JuicerBaseSettings(models.Model):
    max_rpm = model.IntegerField(default=10)
    min_rpm = model.IntegerField(default=0)

应该只有一个实例模型：

There should only be one instance of this model:

juicer_base = JuicerBaseSettings()
juicer_base.save()

当然，如果有人不小心创建了一个新实例，那不是世界末日。我可以做 JuicerBaseSettings.objects.all（）。first（）。但是，有没有一种方法可以锁定它，使得不可能创建多个实例？

Of course, if someone accidentally creates a new instances, it's not the end of the world. I could just do JuicerBaseSettings.objects.all().first(). However, is there a way to lock it down such that it's impossible to create more than 1 instance?

我在SO上发现了两个相关的问题。 此答案建议使用诸如之类的第三方应用程序django-singletons ，似乎没有得到积极维护（最近一次git repo更新是5年前）。 另一个答案建议结合使用权限或 OneToOneField 。这两个答案都来自2010-2011年。

I found two related questions on SO. This answer suggests using 3rd party apps like django-singletons, which doesn't seem to be actively maintained (last update to the git repo is 5 years ago). Another answer suggests using a combination of either permissions or OneToOneField. Both answers are from 2010-2011.

鉴于Django从那时起发生了很大变化，是否有解决此问题的标准方法？还是我应该只使用 .first（）并接受可能存在重复项？

Given that Django has changed a lot since then, are there any standard ways to solve this problem? Or should I just use .first() and accept that there may be duplicates?

推荐答案

您可以重写 save 方法来控制实例数：

You can override save method to control number of instances:

class JuicerBaseSettings(models.Model):

    def save(self, *args, **kwargs):
        if not self.pk and JuicerBaseSettings.objects.exists():
        # if you'll not check for self.pk 
        # then error will also raised in update of exists model
            raise ValidationError('There is can be only one JuicerBaseSettings instance')
        return super(JuicerBaseSettings, self).save(*args, **kwargs)

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