限制ManyToManyField的最大选择 [英] Limit Maximum Choices of ManyToManyField

问题描述

我试图限制模型记录在ManyToManyField中可以选择的最大数量。

I'm trying to limit the maximum amount of choices a model record can have in a ManyToManyField.

在此示例中，有一个BlogSite可以与地区。在此示例中，我想将BlogSite限制为只能具有3个区域。

In this example there is a BlogSite that can be related to Regions. In this example I want to limit the BlogSite to only be able to have 3 regions.

这似乎是之前或之后几次被要求/回答的问题。闲逛了几个小时，我无法找到任何东西。对于这个项目，我正在使用Django 1.3。

This seems like something that would have been asked/answered before, but after a couple hours of poking around I haven't been able to find anything close. For this project, I'm using Django 1.3.

#models.py
class BlogSite(models.Model):
    blog_owner = models.ForeignKey(User)
    site_name = models.CharField(max_length=300)
    region = models.ManyToManyField('Region', blank=True, null=True)
    ....

class Region(models.Model):
    value = models.CharField(max_length=50)
    display_value = models.CharField(max_length=60)
    ....

有什么想法吗？

推荐答案

您可以在 BlogSite 上覆盖 clean 方法

from django.core.exceptions import ValidationError

class BlogSite(models.Model):

    blog_owner = models.ForeignKey(User)
    site_name = models.CharField(max_length=300)
    regions = models.ManyToManyField('Region', blank=True, null=True)

    def clean(self, *args, **kwargs):
        if self.regions.count() > 3:
            raise ValidationError("You can't assign more than three regions")
        super(BlogSite, self).clean(*args, **kwargs)
        #This will not work cause m2m fields are saved after the model is saved

如果使用Django的ModelForm，则会出现此错误在表单的non_field_errors中。

And if you use django's ModelForm then this error will appear in form's non_field_errors.

编辑：

M2m字段在保存模型后保存，因此上面的代码将无法正常工作，可以使用 m2m_changed 信号的正确方法：

M2m fields are saved after the model is saved, so the code above will not work, the correct way you can use m2m_changed signal:

from django.db.models.signals import m2m_changed
from django.core.exceptions import ValidationError


def regions_changed(sender, **kwargs):
    if kwargs['instance'].regions.count() > 3:
        raise ValidationError("You can't assign more than three regions")


m2m_changed.connect(regions_changed, sender=BlogSite.regions.through)

尝试一下对我有用。

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