Checking if selected many-to-many relation fields are disjoint

Question

I'm trying to create a condition that has to be fulfilled before object can be saved to database. I have table A and table B, A has 2 separate many-to-many relations with B. I'm trying to check if condition that these fields treated as sets are disjoint holds before saving an entry.

Here is some code:

class Foo(models.Model):
  first = models.ManyToManyField(Bar, related_name='first')
  second = models.ManyToManyField(Bar, related_name='second')
  def save(self):
    if (set(self.first.all()).isdisjoint(list(self.second.all()))):
      #save
    else:
      #raise exception

But I'm getting error

"< Foo: None >" needs to have a value for field "foo" before this many-to-many relationship can be used.

I'm guessing it wants it to be saved before I do this comparison, but the whole point is not to save it to database before this condition is true. How to do it properly?

Answer 1

Model save method have no access to m2m related fields because it is called before them. If you want to validate them then you should define a custom model form for your FooAdmin class (I suppose you're using django admin) and make this validation there.

class FooForm(forms.ModelForm):
    class Meta:
        model = Foo
        exclude = ()

    def clean():
        cd = self.cleaned_data
        first_objects = cd['first']
        second_objects = cd['second']
        # your logic
        return super(FooForm, self).clean()


class FooAdmin(admin.ModelAdmin):
    form = FooForm