Showing custom model validation exceptions in the Django admin site

Question

I have a booking model that needs to check if the item being booked out is available. I would like to have the logic behind figuring out if the item is available centralised so

Answer 1

In django 1.2, model validation has been added.

You can now add a "clean" method to your models which raise ValidationError exceptions, and it will be called automatically when using the django admin.

The clean() method is called when using the django admin, but NOT called on save().

If you need to use the clean() method outside of the admin, you will need to explicitly call clean() yourself.

http://docs.djangoproject.com/en/dev/ref/models/instances/?from=olddocs#validating-objects

So your clean method could be something like this:

from django.core.exceptions import ValidationError

class MyModel(models.Model):

    def is_available(self):
        #do check here
        return result

    def clean(self):
        if not self.is_available():
            raise ValidationError('Item already booked for those dates')

I haven't made use of it extensively, but seems like much less code than having to create a ModelForm, and then link that form in the admin.py file for use in django admin.