Limit Maximum Choices of ManyToManyField

Question

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

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.

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)
    ....

Any ideas?

Answer 1

You can override clean method on your BlogSite model

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

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

EDIT:

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)

Give it a try it worked for me.