fix: record node children correctly when deserializing

Closes #1479

hugr-py/src/hugr/hugr.py

@@ -669,9 +669,10 @@ def get_meta(idx: int) -> dict[str, Any]:
  parent = None
 
  serial_node.root.parent = -1
- hugr._nodes.append(
- NodeData(serial_node.root.deserialize(), parent, metadata=node_meta)
+ n = hugr._add_node(
+ serial_node.root.deserialize(), parent, metadata=node_meta
  )
+ assert n.idx == idx, "Nodes should be added contiguously"
 
  for (src_node, src_offset), (dst_node, dst_offset) in serial.edges:
  if src_offset is None or dst_offset is None:

hugr-py/tests/serialization/test_basic.py

@@ -1,4 +1,6 @@
+from hugr import Hugr, tys
 from hugr._serialization.serial_hugr import SerialHugr, serialization_version
+from hugr.function import Module
 
 
 def test_empty():
@@ -10,3 +12,15 @@ def test_empty():
  "metadata": None,
  "encoder": None,
  }
+
+
+def test_children():
+ mod = Module()
+ mod.declare_function("foo", tys.PolyFuncType([], tys.FunctionType.empty()))
+
+ h = mod.hugr
+ assert len(h.children(h.root)) == 1
+
+ h2 = Hugr.load_json(h.to_json())
+
+ assert len(h2.children(h2.root)) == 1