from django.db import transaction
from django.db.models.signals import pre_delete
from django.dispatch import receiver

from epdb.models import Node, Edge


@receiver(pre_delete, sender=Node)
@transaction.atomic
def delete_orphan_edges(sender, instance, **kwargs):
    # check if the node that is about to be deleted is the only start node
    for edge in Edge.objects.filter(start_nodes=instance):
        if edge.start_nodes.count() == 1:
            edge.delete()

    # same for end_nodes
    for edge in Edge.objects.filter(end_nodes=instance):
        # check if the node that is about to be deleted is the only start node
        if edge.end_nodes.count() == 1:
            edge.delete()