[Fix] Cascade deletion of Reactions to Edges (#409)

Co-authored-by: Tim Lorsbach <tim@lorsba.ch>
Reviewed-on: enviPath/enviPy#409

epdb/models.py

@@ -2166,7 +2166,7 @@ class Pathway(EnviPathModel, AliasMixin, ScenarioMixin, AdditionalInformationMix
         smiles: str,
         name: Optional[str] = None,
         description: Optional[str] = None,
-        depth: Optional[int] = 0,
+        depth: Optional[int] = -1,
     ):
         return Node.create(self, smiles, depth, name=name, description=description)
 
@@ -2373,7 +2373,7 @@ class Edge(EnviPathModel, AliasMixin, ScenarioMixin, AdditionalInformationMixin)
         "epdb.Pathway", verbose_name="belongs to", on_delete=models.CASCADE, db_index=True
     )
     edge_label = models.ForeignKey(
-        "epdb.Reaction", verbose_name="Edge label", null=True, on_delete=models.SET_NULL
+        "epdb.Reaction", verbose_name="Edge label", null=True, on_delete=models.CASCADE
     )
     start_nodes = models.ManyToManyField(
         "epdb.Node", verbose_name="Start Nodes", related_name="edge_educts"