How to get field with ForeignKey('self') without the possibilty to link to same entry?

Question

Hallo Folks,

I hope you can help me with a little problem I came in touch with while building a model with a foreign key to itself.

Here an example:

class Example (model.Model):
    parent = models.ForeignKey('self', null=True, blank=True)
    # and some other fields

After creating a new entry in the admin panel and going into this example for editing some content, I realize that I could set the parent to the current entry. But thats not what I wanted to get with the ForeignKey and the relation to itself.

Is it possible to disallow the link to itself?

Maybe it's better to use a integer field with the right choices but I'm not quiet sure how to realize this an a smooth and Python like way.

Big thanks
Lemmi

Answer 1

one way to do this would be to override the model's clean method

class Example(model.Model):
    #...
    def clean(self):
        if self.parent.id == self.id:
            raise ValidationError("no self referential models")

this will be called as the second step of object validation and will prevent the object from being inserted in the database.

Answer 2

It is possible to disallow a link from an instance to the same instance. One way to achieve this would be to add a custom model validation. For e.g.

class Example(models.Model):
    ...
    def clean_fields(self):
        if self.id and self.parent.id == self.id:
            raise ValidationError(...)