Fix hard coded pks

This fix enables pytest to run the tests in a shuffled order.
closes #12

Author: Harald Viemann

tests/test.py

@@ -251,8 +251,8 @@ def test_02_dag(self):
         )
         log.debug("Check attributes")
         self.assertEqual(nx_out.graph, {"test": "test"})
-        self.assertEqual(nx_out.nodes[11], {"id": 11, "name": "root"})
-        self.assertEqual(nx_out.edges[11, 14], {"id": 4, "name": "root a3"})
+        self.assertEqual(nx_out.nodes[root.pk], {"id": root.pk, "name": "root"})
+        self.assertEqual(nx_out.edges[root.pk, a3.pk], {"id": NetworkEdge.objects.get(parent=root, child=a3).pk, "name": "root a3"})
 
         """
         Simulate a basic irrigation canal network