You can now add cost items into a cost schedule, and as sub items to a parent cost item. Thanks myoualid and Jesusbill!

src/ifcopenshell-python/ifcopenshell/api/nest/assign_object.py

@@ -0,0 +1,53 @@
+import ifcopenshell
+import ifcopenshell.api
+
+
+class Usecase:
+    def __init__(self, file, **settings):
+        self.file = file
+        self.settings = {
+            "object": None,
+            "relating_object": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        nests = None
+        if self.settings["object"].Nests:
+            nests = self.settings["object"].Nests[0]
+
+        is_nested_by = None
+        for rel in self.settings["relating_object"].IsNestedBy:
+            if rel.is_a("IfcRelNests"):
+                is_nested_by = rel
+                break
+
+        if nests and nests == is_nested_by:
+            return
+
+        if nests:
+            related_objects = list(nests.RelatedObjects)
+            related_objects.remove(self.settings["object"])
+            if related_objects:
+                nests.RelatedObjects = related_objects
+                ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": nests})
+            else:
+                self.file.remove(nests)
+
+        if is_nested_by:
+            related_objects = list(is_nested_by.RelatedObjects)
+            related_objects.append(self.settings["object"])
+            is_nested_by.RelatedObjects = related_objects
+            ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": is_nested_by})
+        else:
+            is_nested_by = self.file.create_entity(
+                "IfcRelNests",
+                **{
+                    "GlobalId": ifcopenshell.guid.new(),
+                    "OwnerHistory": ifcopenshell.api.run("owner.create_owner_history", self.file),
+                    "RelatedObjects": [self.settings["object"]],
+                    "RelatingObject": self.settings["relating_object"],
+                }
+            )
+        return is_nested_by