How to get all ancestors of a Django object when ForeignKey is not self-referencing?

Question

I have a model Person and another model Relation. Before creating a new relation I want to check if this relation is possible or not.

This post and some other similar posts provides a solution but for self-referencing models, my model is not self referencing. Django self-recursive foreignkey filter query for all childs

class Person(models.Model):
    identifier = IntegerField(null=True)
    title = CharField(max_length=100, null=True)

    def get_all_parent_ids(self):
        # I want to get all the ancestors of this person
        # but this method only gets immediate parents right now
        return [parent.parent.id for parent in self.parenting.all()]

    def get_all_children_ids(self):
        # I want to get all the descendants of this person
        # but this method only gets immediate children right now
        return [child.children.id for child in self.baby_sitting.all()]


class Relation(models.Model):
    name = CharField(max_length=50, null=True)
    parent = ForeignKey(Person, on_delete=models.PROTECT, related_name="parenting")
    children = ForeignKey(Person, on_delete=models.PROTECT, related_name="baby_sitting")

    class Meta:
        unique_together = ('parent', 'children')

def is_relation_possible(new_parent_id, new_children_id):
    new_parent_person = Person.objects.get(pk=new_parent_id)
    all_parents = new_parent_person.get_all_parent_ids()
    if new_children_id in all_parents:
        return False
    else:
        return True

For example: Existing relaions - A to B - B to C - C to D - D to E

I want is_relation_possible(E, A) to return False, as E has an ancestor A.

Currently it only check immediate parents and not all parents.

score 0 · Accepted Answer · answered Jul 13 '19 at 12:00

You should to use recursion:

def is_relation_possible(new_parent_id, new_children_id):
    new_parent_person = Person.objects.get(pk=new_parent_id)
    all_parents = new_parent_person.get_all_parent_ids()
    related = ( new_children_id in all_parents 
                or 
                any( is_relation_possible( ancestor_id, new_children_id) 
                     for ancestor_id in all_parents )  # (*1)
               )
    return related

(*1) Here the trick: is related if itself is related or is related through their ancestors.

Notice 1: If you are working with graphs and not with hierarchies, it may turn on an infinite loop.

Notice 2: Not tested. Test it and come back :)

Looks like it works for liner relations at least. Still needs to test it a bit more. I am also trying to make it return a reason why the relation is not possible, like list out only the relevant relations that is making it related. Like this relation is not possible because you already have these relation D -> C -> B -> A — ak4zh, Jul 14 '19 at 11:00

How to get all ancestors of a Django object when ForeignKey is not self-referencing?

1 Answers1