First attempt at ripping out agnostic code into ifcopenshell.api namespace. See #1399.

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

@@ -0,0 +1,53 @@
+import ifcopenshell
+from ifcopenshell.api.owner.api import create_owner_history
+from ifcopenshell.api.owner.api import update_owner_history
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "product": None,
+            "relating_object": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        decomposes = None
+        if self.settings["product"].Decomposes:
+            decomposes = self.settings["product"].Decomposes[0]
+
+        is_decomposed_by = None
+        for rel in self.settings["relating_object"].IsDecomposedBy:
+            if rel.is_a("IfcRelAggregates"):
+                is_decomposed_by = rel
+                break
+
+        if decomposes and decomposes == is_decomposed_by:
+            return
+
+        if decomposes:
+            related_objects = list(decomposes.RelatedObjects)
+            related_objects.remove(self.settings["product"])
+            if related_objects:
+                decomposes.RelatedObjects = related_objects
+                update_owner_history(decomposes)
+            else:
+                self.file.remove(decomposes)
+
+        if is_decomposed_by:
+            related_objects = list(is_decomposed_by.RelatedObjects)
+            related_objects.append(self.settings["product"])
+            is_decomposed_by.RelatedObjects = related_objects
+            update_owner_history(is_decomposed_by)
+        else:
+            is_decomposed_by = self.file.create_entity(
+                "IfcRelAggregates",
+                **{
+                    "GlobalId": ifcopenshell.guid.new(),
+                    "OwnerHistory": create_owner_history(),
+                    "RelatedObjects": [self.settings["product"]],
+                    "RelatingObject": self.settings["relating_object"],
+                }
+            )