Delete Stale Edges when removing a Node from a Pathway (#41)

Co-authored-by: Tim Lorsbach <tim@lorsba.ch>
Reviewed-on: enviPath/enviPy#41

epdb/signals.py

@@ -0,0 +1,20 @@
+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()