Question

I have an AdminModel that allows users to save with any of the fields left empty. However, I don’t want them to be able to save when all fields are empty. I’ve implemented this using the following:

def save_model(self, request, obj, form, change):
    if form.has_changed():
        obj.save()
    else:
        pass

However, the Model has a M2M relationship with another Model, so I get the following error when I hit save:

instance needs to have a primary key value before a many-to-many relationship can be used.

What do I need to add to this method to stop it trying to create the M2M relationship?

Edit:

I’ve added hacky workaround, but I’d be interested in knowing if there’s a better way of doing it. The workaround was to change the else so it changes the ModelAdmin’s field attribute so that it doesn’t contain the field for the M2M relationship – this prevents it from trying to make the relationship. This is not only hacky, but also only works after it’s failed to save the model once and thrown an error…

Answer 1

You can’t – save_m2m is called after save_model. The docs specifically say this hook is not for veto purposes.

ModelAdmin.save_model() and ModelAdmin.delete_model() must save/delete
the object, they are not for veto purposes, rather they allow you to
perform extra operations.

What you need is form validation to prevent save_model from triggering. Somehow, you need to detect when all fields are left blank.

class MyForm(forms.ModelForm):
    class Meta:
        model = MyModel

    def clean(self):
        if not any(self.cleaned_data.values()):
            raise forms.ValidationError("All fields cannot be blank!")
        return self.cleaned_data

class MyAdmin(admin.ModelAdmin):
    form = MyForm