Question

How can I ensure that at least one many to many relation is set?

For example: If I have a listing model which has a image field with a many to many relation to images. How can I ensure that at least one image is set

Bonus question: What if the minimum was something other than one? What about a maximum?

Answer 1

You can implement a function to check if the Listing instance has at least one image instance, and connect that function to the Listing model’s pre_save signal

It’ll be something like, (assuming you are using django 1.3)

from django.db.models.signals import pre_save
from django.dispatch import receiver
from myapp.models import Listing
...
@receiver(pre_save, sender=Listing)
def check_image_requirement(sender, instance, **kwargs):
    if instance.images.count() == 0:
        raise your_own_exception("Listing is required to have at least one image")

where you need to implement your_own_exception

The following addition is the response to PO’s further questions

Implementing Listing.clean() is another way to achieve the same validation rule. Indeed, it’s the semantically correct approach as Model.clean() is meant for custom model validations. But adopting this approach would be less convenient – to trigger the clean() you would have to either manually call full_clean() (if you don’t use model form), or manually call is_valid() (when using model form), right before calling save() of a Listing instance. Reference

On the other hand, with the pre_save signal approach, you can be certain that the validation rule is always applied on Listing instance whenever you call save() on the instance.

It’s not a right-or-wrong to choose one over the other but merely a design decision to make. Both approaches can achieve what you need and keep the business/domain logic (ie. your validation rule) in the Models layer.