fix and add test (#706)

molecularnodes/blender/nodes.py

@@ -328,6 +328,13 @@ def create_starting_nodes_density(object, threshold=0.8, style="density_surface"
     mod = get_mod(object)
     node_name = f"MN_density_{object.name}"
 
+    try:
+        tree = bpy.data.node_groups[node_name]
+        mod.node_group = tree
+        return
+    except KeyError:
+        pass
+
     # create a new GN node group, specific to this particular molecule
     group = new_tree(node_name, fallback=False)
     link = group.links.new
@@ -380,8 +387,14 @@ def create_starting_node_tree(
     if not name:
         name = f"MN_{object.name}"
 
-    # create a new GN node group, specific to this particular molecule
-    mod = get_mod(object)
+    # check if the node tree already exists and use that instead
+    try:
+        tree = bpy.data.node_groups[name]
+        mod.node_group = tree
+        return
+    except KeyError:
+        pass
+
     tree = new_tree(name)
     tree.is_modifier = is_modifier
     link = tree.links.new

tests/test_nodes.py

@@ -283,3 +283,16 @@ def test_is_modifier():
             assert not tree.is_modifier
     mol = mn.entities.fetch("4ozs")
     assert mol.tree.is_modifier
+
+
+def test_reuse_node_group():
+    mol = mn.entities.fetch("4ozs")
+    tree = bpy.data.node_groups["MN_4ozs"]
+    n_nodes = len(tree.nodes)
+    bpy.data.objects.remove(mol.object)
+    del mol
+
+    assert n_nodes == len(tree.nodes)
+
+    mol = mn.fetch("4ozs")
+    assert n_nodes == len(tree.nodes)