diff --git a/src/blenderbim/blenderbim/bim/module/structural/operator.py b/src/blenderbim/blenderbim/bim/module/structural/operator.py
index 3662397056..47012097c3 100644
--- a/src/blenderbim/blenderbim/bim/module/structural/operator.py
+++ b/src/blenderbim/blenderbim/bim/module/structural/operator.py
@@ -473,7 +473,7 @@ class AssignStructuralLoadCase(bpy.types.Operator, tool.Ifc.Operator):
             self.file,
             **{
                 "relating_object": self.file.by_id(self.work_plan),
-                "product": self.file.by_id(self.load_case),
+                "products": [self.file.by_id(self.load_case)],
             },
         )
         return {"FINISHED"}
diff --git a/src/blenderbim/blenderbim/core/aggregate.py b/src/blenderbim/blenderbim/core/aggregate.py
index e74f8d7bb5..09c6f1c4e6 100644
--- a/src/blenderbim/blenderbim/core/aggregate.py
+++ b/src/blenderbim/blenderbim/core/aggregate.py
@@ -29,7 +29,7 @@ def assign_object(ifc, aggregator, collector, relating_obj=None, related_obj=Non
     if not aggregator.can_aggregate(relating_obj, related_obj):
         return
     rel = ifc.run(
-        "aggregate.assign_object", product=ifc.get_entity(related_obj), relating_object=ifc.get_entity(relating_obj)
+        "aggregate.assign_object", products=[ifc.get_entity(related_obj)], relating_object=ifc.get_entity(relating_obj)
     )
     collector.assign(relating_obj)
     collector.assign(related_obj)
diff --git a/src/blenderbim/blenderbim/core/sequence.py b/src/blenderbim/blenderbim/core/sequence.py
index 2fa2debf5d..970b05e38d 100644
--- a/src/blenderbim/blenderbim/core/sequence.py
+++ b/src/blenderbim/blenderbim/core/sequence.py
@@ -61,7 +61,7 @@ def remove_work_schedule(ifc, work_schedule=None):
 
 def assign_work_schedule(ifc, work_plan=None, work_schedule=None):
     if work_schedule:
-        return ifc.run("aggregate.assign_object", relating_object=work_plan, product=work_schedule)
+        return ifc.run("aggregate.assign_object", relating_object=work_plan, products=[work_schedule])
 
 
 def unassign_work_schedule(ifc, work_schedule=None):
diff --git a/src/blenderbim/scripts/obj2ifc-meshlab.py b/src/blenderbim/scripts/obj2ifc-meshlab.py
index 57d658ad90..2ecb5684fb 100644
--- a/src/blenderbim/scripts/obj2ifc-meshlab.py
+++ b/src/blenderbim/scripts/obj2ifc-meshlab.py
@@ -127,9 +127,9 @@ class Obj2Ifc:
         self.storey = ifcopenshell.api.run(
             "root.create_entity", self.file, ifc_class="IfcBuildingStorey", name="My Storey"
         )
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=site, relating_object=project)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=building, relating_object=site)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=self.storey, relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[site], relating_object=project)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[building], relating_object=site)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[self.storey], relating_object=building)
 
         ifcopenshell.api.run("geometry.edit_object_placement", self.file, product=site)
         ifcopenshell.api.run("geometry.edit_object_placement", self.file, product=building)
diff --git a/src/blenderbim/scripts/obj2ifc.py b/src/blenderbim/scripts/obj2ifc.py
index b056c9539b..2999fbb6b6 100644
--- a/src/blenderbim/scripts/obj2ifc.py
+++ b/src/blenderbim/scripts/obj2ifc.py
@@ -120,9 +120,9 @@ class Obj2Ifc:
         self.storey = ifcopenshell.api.run(
             "root.create_entity", self.file, ifc_class="IfcBuildingStorey", name="My Storey"
         )
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=site, relating_object=project)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=building, relating_object=site)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=self.storey, relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[site], relating_object=project)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[building], relating_object=site)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[self.storey], relating_object=building)
 
         ifcopenshell.api.run("geometry.edit_object_placement", self.file, product=site)
         ifcopenshell.api.run("geometry.edit_object_placement", self.file, product=building)
diff --git a/src/blenderbim/test/core/test_aggregate.py b/src/blenderbim/test/core/test_aggregate.py
index 26de9166a1..2458e48e40 100644
--- a/src/blenderbim/test/core/test_aggregate.py
+++ b/src/blenderbim/test/core/test_aggregate.py
@@ -38,7 +38,7 @@ class TestAssignObject:
         ifc.get_entity("relating_obj").should_be_called().will_return("relating_object")
         ifc.get_entity("related_obj").should_be_called().will_return("related_object")
         ifc.run(
-            "aggregate.assign_object", product="related_object", relating_object="relating_object"
+            "aggregate.assign_object", products=["related_object"], relating_object="relating_object"
         ).should_be_called().will_return("rel")
         aggregate.disable_editing("related_obj").should_be_called()
         collector.assign("relating_obj").should_be_called()
diff --git a/src/blenderbim/test/tool/test_brick.py b/src/blenderbim/test/tool/test_brick.py
index b70b9b2240..b5d5a1f973 100644
--- a/src/blenderbim/test/tool/test_brick.py
+++ b/src/blenderbim/test/tool/test_brick.py
@@ -320,8 +320,8 @@ class TestGetParentSpace(NewFile):
         element = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcBuildingStorey")
         subelement = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcSpace")
         project = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcProject")
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=subelement, relating_object=element)
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=element, relating_object=project)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[subelement], relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[element], relating_object=project)
         assert subject.get_parent_space(subelement) == element
         assert subject.get_parent_space(element) is None
 
diff --git a/src/blenderbim/test/tool/test_collector.py b/src/blenderbim/test/tool/test_collector.py
index 1d9353b75b..f9bf18d0f1 100644
--- a/src/blenderbim/test/tool/test_collector.py
+++ b/src/blenderbim/test/tool/test_collector.py
@@ -68,7 +68,7 @@ class TestAssign(NewFile):
             "aggregate.assign_object",
             tool.Ifc.get(),
             relating_object=tool.Ifc.get().by_type("IfcSite")[0],
-            product=space_element,
+            products=[space_element],
         )
         subject.assign(space_obj)
         assert len(space_obj.users_collection) == 1
@@ -84,7 +84,7 @@ class TestAssign(NewFile):
             "aggregate.assign_object",
             tool.Ifc.get(),
             relating_object=tool.Ifc.get().by_type("IfcSite")[0],
-            product=space_element,
+            products=[space_element],
         )
         subject.assign(space_obj)
         subject.assign(space_obj)
@@ -103,7 +103,7 @@ class TestAssign(NewFile):
             "aggregate.assign_object",
             tool.Ifc.get(),
             relating_object=tool.Ifc.get().by_type("IfcSite")[0],
-            product=space_element,
+            products=[space_element],
         )
         subject.assign(space_obj)
         assert len(space_obj.users_collection) == 1
@@ -121,7 +121,7 @@ class TestAssign(NewFile):
             "aggregate.assign_object",
             tool.Ifc.get(),
             relating_object=element,
-            product=subelement,
+            products=[subelement],
         )
         subject.assign(element_obj)
         assert len(element_obj.users_collection) == 1
@@ -161,7 +161,7 @@ class TestAssign(NewFile):
             "aggregate.assign_object",
             tool.Ifc.get(),
             relating_object=tool.Ifc.get().by_type("IfcSite")[0],
-            product=space_element,
+            products=[space_element],
         )
         subject.assign(space_obj)
         assert bpy.context.scene.collection.children.find(space_collection.name) == -1
@@ -180,7 +180,7 @@ class TestAssign(NewFile):
             "aggregate.assign_object",
             tool.Ifc.get(),
             relating_object=element,
-            product=subelement,
+            products=[subelement],
         )
         subject.assign(element_obj)
         subject.assign(subelement_obj)
@@ -203,7 +203,7 @@ class TestAssign(NewFile):
             "aggregate.assign_object",
             tool.Ifc.get(),
             relating_object=element,
-            product=subelement,
+            products=[subelement],
         )
         subject.assign(subelement_obj)
         assert subelement_obj.users_collection[0].name == "IfcSite/My Site"
diff --git a/src/blenderbim/test/tool/test_misc.py b/src/blenderbim/test/tool/test_misc.py
index f5dab4a626..bf7e0f5ce7 100644
--- a/src/blenderbim/test/tool/test_misc.py
+++ b/src/blenderbim/test/tool/test_misc.py
@@ -86,8 +86,8 @@ class TestGetStoreyHeight(test.bim.bootstrap.NewFile):
         storey.Elevation = 3000
         storey2 = ifc.createIfcBuildingStorey()
         storey2.Elevation = 5000
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=storey, relating_object=building)
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=storey2, relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[storey], relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[storey2], relating_object=building)
         assert subject.get_storey_height_in_si(storey, 1) == 2.0
 
     def test_getting_a_double_storey_height(self):
@@ -103,9 +103,9 @@ class TestGetStoreyHeight(test.bim.bootstrap.NewFile):
         storey2.Elevation = 5000
         storey3 = ifc.createIfcBuildingStorey()
         storey3.Elevation = 9000
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=storey, relating_object=building)
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=storey2, relating_object=building)
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=storey3, relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[storey], relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[storey2], relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[storey3], relating_object=building)
         assert subject.get_storey_height_in_si(storey, 2) == 6.0
 
     def test_only_considering_storeys_in_the_same_building(self):
@@ -116,7 +116,7 @@ class TestGetStoreyHeight(test.bim.bootstrap.NewFile):
         storey.Elevation = 3000
         storey2 = ifc.createIfcBuildingStorey()
         storey2.Elevation = 5000
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=storey, relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[storey], relating_object=building)
         assert subject.get_storey_height_in_si(storey, 1) is None
 
     def test_returning_none_if_the_storey_height_is_undefined(self):
@@ -124,7 +124,7 @@ class TestGetStoreyHeight(test.bim.bootstrap.NewFile):
         tool.Ifc.set(ifc)
         building = ifc.createIfcBuilding()
         storey = ifc.createIfcBuildingStorey()
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=storey, relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[storey], relating_object=building)
         assert subject.get_storey_height_in_si(storey, 1) is None
 
 
diff --git a/src/ifcopenshell-python/docs/ifcopenshell-python/code_examples.rst b/src/ifcopenshell-python/docs/ifcopenshell-python/code_examples.rst
index 66f708e95e..b43652b975 100644
--- a/src/ifcopenshell-python/docs/ifcopenshell-python/code_examples.rst
+++ b/src/ifcopenshell-python/docs/ifcopenshell-python/code_examples.rst
@@ -236,9 +236,9 @@ Create a simple model from scratch
 
     # Since the site is our top level location, assign it to the project
     # Then place our building on the site, and our storey in the building
-    run("aggregate.assign_object", model, relating_object=project, product=site)
-    run("aggregate.assign_object", model, relating_object=site, product=building)
-    run("aggregate.assign_object", model, relating_object=building, product=storey)
+    run("aggregate.assign_object", model, relating_object=project, products=[site])
+    run("aggregate.assign_object", model, relating_object=site, products=[building])
+    run("aggregate.assign_object", model, relating_object=building, products=[storey])
 
     # Let's create a new wall
     wall = run("root.create_entity", model, ifc_class="IfcWall")
diff --git a/src/ifcopenshell-python/ifcopenshell/api/__init__.py b/src/ifcopenshell-python/ifcopenshell/api/__init__.py
index 905ebeaaf4..376fd14f3f 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/__init__.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/__init__.py
@@ -50,6 +50,7 @@ ARGUMENTS_DEPRECATION = {
         batching_argument_deprecation, prev_argument="product", new_argument="products"
     ),
     "group.unassign_group": partial(batching_argument_deprecation, prev_argument="product", new_argument="products"),
+    "aggregate.assign_object": partial(batching_argument_deprecation, prev_argument="product", new_argument="products"),
     "layer.assign_layer": partial(batching_argument_deprecation, prev_argument="item", new_argument="items"),
     "layer.unassign_layer": partial(batching_argument_deprecation, prev_argument="item", new_argument="items"),
 }
diff --git a/src/ifcopenshell-python/ifcopenshell/api/aggregate/assign_object.py b/src/ifcopenshell-python/ifcopenshell/api/aggregate/assign_object.py
index 8a868755be..58bf9daa13 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/aggregate/assign_object.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/aggregate/assign_object.py
@@ -20,11 +20,17 @@ import ifcopenshell
 import ifcopenshell.api
 import ifcopenshell.util.element
 import ifcopenshell.util.placement
+from typing import Union
 
 
 class Usecase:
-    def __init__(self, file, product=None, relating_object=None):
-        """Assigns an object as an aggregate to a product
+    def __init__(
+        self,
+        file: ifcopenshell.file,
+        products: list[ifcopenshell.entity_instance],
+        relating_object: ifcopenshell.entity_instance,
+    ):
+        """Assigns object as an aggregate to the products
 
         All physical IFC model elements must be part of a hierarchical tree
         called the "spatial decomposition", where large things are made up of
@@ -55,14 +61,15 @@ class Usecase:
         its parent in the spatial hierarchy. If your product has a placement,
         its placement will be recalculated to follow this convention.
 
-        :param product: The part of the aggregate, typically an IfcElement or
+        :param products: The list of parts of the aggregate, typically of IfcElement or
             IfcSpatialStructureElement subclass
-        :type product: ifcopenshell.entity_instance.entity_instance
+        :type product: list[ifcopenshell.entity_instance.entity_instance]
         :param relating_object: The whole of the aggregate, typically an
             IfcElement or IfcSpatialStructureElement subclass
         :type relating_object: ifcopenshell.entity_instance.entity_instance
         :return: The IfcRelAggregate relationship instance
-        :rtype: ifcopenshell.entity_instance.entity_instance
+            or `None` if `products` was empty list.
+        :rtype: Union[ifcopenshell.entity_instance.entity_instance, None]
 
         Example:
 
@@ -73,40 +80,59 @@ class Usecase:
             subelement = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcBuilding")
 
             # The project contains a site (note that project aggregation is a special case in IFC)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=element, relating_object=project)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[element], relating_object=project)
 
             # The site has a building
-            ifcopenshell.api.run("aggregate.assign_object", model, product=subelement, relating_object=element)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[subelement], relating_object=element)
         """
         self.file = file
         self.settings = {
-            "product": product,
+            "products": products,
             "relating_object": relating_object,
         }
 
-    def execute(self):
-        decomposes = None
-        if self.settings["product"].Decomposes:
-            decomposes = self.settings["product"].Decomposes[0]
+    def execute(self) -> Union[ifcopenshell.entity_instance, None]:
+        if not self.settings["products"]:
+            return
 
-        is_decomposed_by = None
-        for rel in self.settings["relating_object"].IsDecomposedBy:
-            if rel.is_a("IfcRelAggregates"):
-                is_decomposed_by = rel
-                break
+        products = set(self.settings["products"])
+        relating_object = self.settings["relating_object"]
+        is_decomposed_by = next((i for i in relating_object.IsDecomposedBy if i.is_a("IfcRelAggregates")), None)
 
-        if decomposes and decomposes == is_decomposed_by:
-            return decomposes
+        previous_aggregates_rels: set[ifcopenshell.entity_instance] = set()
+        products_without_aggregates: list[ifcopenshell.entity_instance] = []
+        products_with_aggregates: list[ifcopenshell.entity_instance] = []
 
-        container = ifcopenshell.util.element.get_container(self.settings["product"], should_get_direct=True)
-        if container:
-            ifcopenshell.api.run("spatial.remove_container", self.file, product=self.settings["product"])
+        # check if there is anything to change
+        for product in products:
+            product_rel = next(iter(product.Decomposes), None)
 
-        if decomposes:
-            related_objects = list(decomposes.RelatedObjects)
-            related_objects.remove(self.settings["product"])
+            if product_rel is None:
+                products_without_aggregates.append(product)
+                continue
+
+            # either is_decomposed_by is None or product is part of different rel
+            if product_rel != is_decomposed_by:
+                previous_aggregates_rels.add(product_rel)
+                products_with_aggregates.append(product)
+
+            # products with already assigned aggregates will be skipped
+
+        products_to_change = products_without_aggregates + products_with_aggregates
+        # nothing to change
+        if not products_to_change:
+            return is_decomposed_by
+
+        # can be either only aggregated or only contained at the same time
+        # some product might not be able to have a container
+        possibly_contained_products = [p for p in products_without_aggregates if hasattr(p, "ContainedInStructure")]
+        ifcopenshell.api.run("spatial.unassign_container", self.file, products=possibly_contained_products)
+
+        # unassign elements from previous aggregates
+        for decomposes in previous_aggregates_rels:
+            related_objects = set(decomposes.RelatedObjects) - products
             if related_objects:
-                decomposes.RelatedObjects = related_objects
+                decomposes.RelatedObjects = list(related_objects)
                 ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": decomposes})
             else:
                 history = decomposes.OwnerHistory
@@ -114,10 +140,9 @@ class Usecase:
                 if history:
                     ifcopenshell.util.element.remove_deep2(self.file, history)
 
+        # assign elements to a new aggregate
         if is_decomposed_by:
-            related_objects = set(is_decomposed_by.RelatedObjects)
-            related_objects.add(self.settings["product"])
-            is_decomposed_by.RelatedObjects = list(related_objects)
+            is_decomposed_by.RelatedObjects = list(set(is_decomposed_by.RelatedObjects) | products)
             ifcopenshell.api.run("owner.update_owner_history", self.file, **{"element": is_decomposed_by})
         else:
             is_decomposed_by = self.file.create_entity(
@@ -125,19 +150,21 @@ class Usecase:
                 **{
                     "GlobalId": ifcopenshell.guid.new(),
                     "OwnerHistory": ifcopenshell.api.run("owner.create_owner_history", self.file),
-                    "RelatedObjects": [self.settings["product"]],
-                    "RelatingObject": self.settings["relating_object"],
+                    "RelatedObjects": list(products),
+                    "RelatingObject": relating_object,
                 }
             )
 
-        placement = getattr(self.settings["product"], "ObjectPlacement", None)
-        if placement and placement.is_a("IfcLocalPlacement"):
-            ifcopenshell.api.run(
-                "geometry.edit_object_placement",
-                self.file,
-                product=self.settings["product"],
-                matrix=ifcopenshell.util.placement.get_local_placement(self.settings["product"].ObjectPlacement),
-                is_si=False,
-            )
+        # localize placement relative to a new aggregate for affected products
+        for product in products_to_change:
+            placement = getattr(product, "ObjectPlacement", None)
+            if placement and placement.is_a("IfcLocalPlacement"):
+                ifcopenshell.api.run(
+                    "geometry.edit_object_placement",
+                    self.file,
+                    product=product,
+                    matrix=ifcopenshell.util.placement.get_local_placement(product.ObjectPlacement),
+                    is_si=False,
+                )
 
         return is_decomposed_by
diff --git a/src/ifcopenshell-python/ifcopenshell/api/aggregate/unassign_object.py b/src/ifcopenshell-python/ifcopenshell/api/aggregate/unassign_object.py
index b4c4c42a79..f92b9e60cf 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/aggregate/unassign_object.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/aggregate/unassign_object.py
@@ -51,8 +51,8 @@ class Usecase:
             element = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcSite")
             subelement1 = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcBuilding")
             subelement2 = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcBuilding")
-            ifcopenshell.api.run("aggregate.assign_object", model, product=subelement1, relating_object=element)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=subelement2, relating_object=element)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[subelement1], relating_object=element)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[subelement2], relating_object=element)
             # The relationship is returned as element still has subelement2
             rel = ifcopenshell.api.run("aggregate.unassign_object", model, product=subelement1)
             # Nothing is returned, as element is now empty
diff --git a/src/ifcopenshell-python/ifcopenshell/api/sequence/add_work_schedule.py b/src/ifcopenshell-python/ifcopenshell/api/sequence/add_work_schedule.py
index a9c775d338..3da4d3b10e 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/add_work_schedule.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/add_work_schedule.py
@@ -107,7 +107,7 @@ class Usecase:
                 "aggregate.assign_object",
                 self.file,
                 **{
-                    "product": work_schedule,
+                    "products": [work_schedule],
                     "relating_object": self.settings["work_plan"],
                 }
             )
diff --git a/src/ifcopenshell-python/ifcopenshell/api/sequence/assign_workplan.py b/src/ifcopenshell-python/ifcopenshell/api/sequence/assign_workplan.py
index b3439e69a6..a9e6f000eb 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/sequence/assign_workplan.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/sequence/assign_workplan.py
@@ -64,7 +64,7 @@ class Usecase:
             "aggregate.assign_object",
             self.file,
             **{
-                "product": self.settings["work_schedule"],
+                "products": [self.settings["work_schedule"]],
                 "relating_object": self.settings["work_plan"],
             }
         )
diff --git a/src/ifcopenshell-python/ifcopenshell/api/spatial/assign_container.py b/src/ifcopenshell-python/ifcopenshell/api/spatial/assign_container.py
index 41114e4b1d..2fee48d67c 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/spatial/assign_container.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/spatial/assign_container.py
@@ -73,7 +73,7 @@ class Usecase:
             as IfcBuilding, IfcBuildingStorey, or IfcSpace that the element
             exists in.
         :return: The IfcRelContainedInSpatialStructure relationship instance
-            or `None` if nothing was changed.
+            or `None` if `products` was empty list.
         :rtype: Union[ifcopenshell.entity_instance.entity_instance, None]
 
         Example:
@@ -128,13 +128,17 @@ class Usecase:
                 products_without_containers.append(product)
                 continue
 
+            # either structure_rel is None or product is part of different rel
             if product_rel != structure_rel:
                 previous_containers_rels.add(product_rel)
                 products_with_containers.append(product)
 
+            # products with already assigned containers will be skipped
+
+        products_to_change = products_without_containers + products_with_containers
         # nothing to change
-        if not products_without_containers and not products_with_containers:
-            return
+        if not products_to_change:
+            return structure_rel
 
         # can be either only aggregated or only contained at the same time
         for product in products_without_containers:
@@ -142,7 +146,7 @@ class Usecase:
             if aggregate:
                 ifcopenshell.api.run("aggregate.unassign_object", self.file, product=product)
 
-        # unassign elements from previous container
+        # unassign elements from previous containers
         for rel in previous_containers_rels:
             related_elements = set(rel.RelatedElements) - products
             if related_elements:
@@ -170,7 +174,7 @@ class Usecase:
             )
 
         # localize placement relative to a new container for affected products
-        for product in products_without_containers + products_with_containers:
+        for product in products_to_change:
             placement = getattr(product, "ObjectPlacement", None)
             if placement and placement.is_a("IfcLocalPlacement"):
                 ifcopenshell.api.run(
diff --git a/src/ifcopenshell-python/ifcopenshell/api/spatial/dereference_structure.py b/src/ifcopenshell-python/ifcopenshell/api/spatial/dereference_structure.py
index 1b492634f0..4a62532762 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/spatial/dereference_structure.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/spatial/dereference_structure.py
@@ -45,12 +45,12 @@ class Usecase:
             storey3 = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcBuildingStorey")
 
             # The project contains a site (note that project aggregation is a special case in IFC)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=site, relating_object=project)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[site], relating_object=project)
 
             # The site has a building, the building has a storey, and the storey has a space
-            ifcopenshell.api.run("aggregate.assign_object", model, product=building, relating_object=site)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=storey, relating_object=building)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=space, relating_object=storey)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[building], relating_object=site)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[storey], relating_object=building)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[space], relating_object=storey)
 
             # Create a column, this column spans 3 storeys
             column = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWall")
diff --git a/src/ifcopenshell-python/ifcopenshell/api/spatial/reference_structure.py b/src/ifcopenshell-python/ifcopenshell/api/spatial/reference_structure.py
index ab20b6aa03..187ce20afe 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/spatial/reference_structure.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/spatial/reference_structure.py
@@ -59,12 +59,12 @@ class Usecase:
             storey3 = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcBuildingStorey")
 
             # The project contains a site (note that project aggregation is a special case in IFC)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=site, relating_object=project)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[site], relating_object=project)
 
             # The site has a building, the building has a storey, and the storey has a space
-            ifcopenshell.api.run("aggregate.assign_object", model, product=building, relating_object=site)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=storey, relating_object=building)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=space, relating_object=storey)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[building], relating_object=site)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[storey], relating_object=building)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[space], relating_object=storey)
 
             # Create a column, this column spans 3 storeys
             column = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWall")
diff --git a/src/ifcopenshell-python/ifcopenshell/api/spatial/remove_container.py b/src/ifcopenshell-python/ifcopenshell/api/spatial/remove_container.py
index 476dd46315..c2c083bd06 100644
--- a/src/ifcopenshell-python/ifcopenshell/api/spatial/remove_container.py
+++ b/src/ifcopenshell-python/ifcopenshell/api/spatial/remove_container.py
@@ -42,11 +42,11 @@ class Usecase:
             storey = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcBuildingStorey")
 
             # The project contains a site (note that project aggregation is a special case in IFC)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=site, relating_object=project)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[site], relating_object=project)
 
             # The site has a building, the building has a storey, and the storey has a space
-            ifcopenshell.api.run("aggregate.assign_object", model, product=building, relating_object=site)
-            ifcopenshell.api.run("aggregate.assign_object", model, product=storey, relating_object=building)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[building], relating_object=site)
+            ifcopenshell.api.run("aggregate.assign_object", model, products=[storey], relating_object=building)
 
             # Create a wall
             wall = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWall")
diff --git a/src/ifcopenshell-python/test/api/aggregate/test_assign_object.py b/src/ifcopenshell-python/test/api/aggregate/test_assign_object.py
index 88b93bfd82..c43733f426 100644
--- a/src/ifcopenshell-python/test/api/aggregate/test_assign_object.py
+++ b/src/ifcopenshell-python/test/api/aggregate/test_assign_object.py
@@ -25,19 +25,23 @@ import ifcopenshell.util.placement
 
 
 class TestAssignObject(test.bootstrap.IFC4):
-    def test_assigning_a_container(self):
+    def test_assigning_an_aggregate(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
-        subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
-        rel = ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
-        assert ifcopenshell.util.element.get_aggregate(subelement) == element
+        subelement1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
+        subelement2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
+        rel = ifcopenshell.api.run(
+            "aggregate.assign_object", self.file, products=[subelement1, subelement2], relating_object=element
+        )
+        assert ifcopenshell.util.element.get_aggregate(subelement1) == element
+        assert ifcopenshell.util.element.get_aggregate(subelement2) == element
         assert rel.is_a("IfcRelAggregates")
 
     def test_doing_nothing_if_the_aggregate_is_already_assigned(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         total_elements = len([e for e in self.file])
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         assert len([e for e in self.file]) == total_elements
 
     def test_that_old_aggregate_relationships_are_updated_if_they_still_have_elements(self):
@@ -45,20 +49,20 @@ class TestAssignObject(test.bootstrap.IFC4):
         element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
         subelement1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         subelement2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement1, relating_object=element1)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement2, relating_object=element1)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement1], relating_object=element1)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement2], relating_object=element1)
         rel = subelement1.Decomposes[0]
         assert len(rel.RelatedObjects) == 2
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement1, relating_object=element2)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement1], relating_object=element2)
         assert len(rel.RelatedObjects) == 1
 
     def test_that_old_aggregate_relationships_are_purged_if_no_more_elements_are_contained(self):
         element1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
         element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
         subelement1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement1, relating_object=element1)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement1], relating_object=element1)
         rel_id = subelement1.Decomposes[0].id()
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement1, relating_object=element2)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement1], relating_object=element2)
         with pytest.raises(RuntimeError):
             self.file.by_id(rel_id)
 
@@ -68,7 +72,7 @@ class TestAssignObject(test.bootstrap.IFC4):
         element1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
         element2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element1)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element1)
         matrix1 = numpy.array(
             (
                 (1.0, 0.0, 0.0, 1.0),
@@ -94,7 +98,7 @@ class TestAssignObject(test.bootstrap.IFC4):
         ifcopenshell.api.run(
             "geometry.edit_object_placement", self.file, product=subelement, matrix=matrix1.copy(), is_si=False
         )
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element2)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element2)
         assert subelement.ObjectPlacement.PlacementRelTo.PlacesObject[0] == element2
         assert numpy.array_equal(ifcopenshell.util.placement.get_local_placement(subelement.ObjectPlacement), matrix1)
 
@@ -105,7 +109,7 @@ class TestAssignObject(test.bootstrap.IFC4):
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         placement = self.file.createIfcGridPlacement()
         subelement.ObjectPlacement = placement
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         assert subelement.ObjectPlacement == placement
 
     def test_removing_containment_if_it_exists(self):
@@ -115,5 +119,5 @@ class TestAssignObject(test.bootstrap.IFC4):
         container = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuildingStorey")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         ifcopenshell.api.run("spatial.assign_container", self.file, products=[subelement], relating_structure=container)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         assert not ifcopenshell.util.element.get_container(subelement, should_get_direct=True)
diff --git a/src/ifcopenshell-python/test/api/aggregate/test_unassign_object.py b/src/ifcopenshell-python/test/api/aggregate/test_unassign_object.py
index de44b0727e..343fe7deed 100644
--- a/src/ifcopenshell-python/test/api/aggregate/test_unassign_object.py
+++ b/src/ifcopenshell-python/test/api/aggregate/test_unassign_object.py
@@ -26,7 +26,7 @@ class TestUnassignObject(test.bootstrap.IFC4):
     def test_unassigning_an_object(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         ifcopenshell.api.run("aggregate.unassign_object", self.file, product=subelement)
         assert ifcopenshell.util.element.get_aggregate(subelement) is None
 
@@ -34,14 +34,14 @@ class TestUnassignObject(test.bootstrap.IFC4):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
         subelement2 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         subelement1 = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement1, relating_object=element)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement2, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement1], relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement2], relating_object=element)
         ifcopenshell.api.run("aggregate.unassign_object", self.file, product=subelement1)
         assert len(self.file.by_type("IfcRelAggregates")) == 1
 
     def test_the_rel_is_purged_if_there_are_no_more_decomposed_elements(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         ifcopenshell.api.run("aggregate.unassign_object", self.file, product=subelement)
         assert len(self.file.by_type("IfcRelAggregates")) == 0
diff --git a/src/ifcopenshell-python/test/api/geometry/test_edit_object_placement.py b/src/ifcopenshell-python/test/api/geometry/test_edit_object_placement.py
index 8c9de31134..73985fd627 100644
--- a/src/ifcopenshell-python/test/api/geometry/test_edit_object_placement.py
+++ b/src/ifcopenshell-python/test/api/geometry/test_edit_object_placement.py
@@ -253,7 +253,7 @@ class TestEditObjectPlacement(test.bootstrap.IFC4):
                 (0.0, 0.0, 0.0, 1.0),
             )
         )
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         ifcopenshell.api.run(
             "geometry.edit_object_placement", self.file, product=element, matrix=matrix.copy(), is_si=False
         )
@@ -629,7 +629,7 @@ class TestEditObjectPlacement(test.bootstrap.IFC4):
 
         building = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         storey = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuildingStorey")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=storey, relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[storey], relating_object=building)
         wall = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         ifcopenshell.api.run("spatial.assign_container", self.file, products=[wall], relating_structure=storey)
 
diff --git a/src/ifcopenshell-python/test/api/root/test_copy_class.py b/src/ifcopenshell-python/test/api/root/test_copy_class.py
index f8c42cdb74..5fcaf8cb90 100644
--- a/src/ifcopenshell-python/test/api/root/test_copy_class.py
+++ b/src/ifcopenshell-python/test/api/root/test_copy_class.py
@@ -89,7 +89,7 @@ class TestCopyClass(test.bootstrap.IFC4):
     def test_copying_a_container_only_and_not_its_decomposition(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuildingStorey")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         new = ifcopenshell.api.run("root.copy_class", self.file, product=element)
         assert element.IsDecomposedBy
         assert not new.IsDecomposedBy
@@ -97,7 +97,7 @@ class TestCopyClass(test.bootstrap.IFC4):
     def test_copying_an_aggregate_only_and_not_its_decomposition(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcElementAssembly")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBeam")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         new = ifcopenshell.api.run("root.copy_class", self.file, product=element)
         assert element.IsDecomposedBy
         assert not new.IsDecomposedBy
@@ -105,7 +105,7 @@ class TestCopyClass(test.bootstrap.IFC4):
     def test_copying_an_aggregate_decomposition_and_maintaining_the_aggregate_relationship(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcElementAssembly")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBeam")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         new = ifcopenshell.api.run("root.copy_class", self.file, product=subelement)
         assert new.Decomposes[0].RelatingObject == element
 
diff --git a/src/ifcopenshell-python/test/api/root/test_remove_product.py b/src/ifcopenshell-python/test/api/root/test_remove_product.py
index 4913afd23c..68ddc40a58 100644
--- a/src/ifcopenshell-python/test/api/root/test_remove_product.py
+++ b/src/ifcopenshell-python/test/api/root/test_remove_product.py
@@ -212,7 +212,7 @@ class TestRemoveProduct(test.bootstrap.IFC4):
     def test_removing_all_aggregate_relationships_of_a_whole(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcElementAssembly")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBeam")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         total_entities = len(list(self.file))
         ifcopenshell.api.run("root.remove_product", self.file, product=element)
         assert len(list(self.file)) == total_entities - 2
@@ -223,7 +223,7 @@ class TestRemoveProduct(test.bootstrap.IFC4):
     def test_removing_all_aggregate_relationships_of_a_part(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcElementAssembly")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBeam")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         total_entities = len(list(self.file))
         ifcopenshell.api.run("root.remove_product", self.file, product=subelement)
         assert len(list(self.file)) == total_entities - 2
diff --git a/src/ifcopenshell-python/test/api/spatial/test_assign_container.py b/src/ifcopenshell-python/test/api/spatial/test_assign_container.py
index 26315c8ec2..533db20267 100644
--- a/src/ifcopenshell-python/test/api/spatial/test_assign_container.py
+++ b/src/ifcopenshell-python/test/api/spatial/test_assign_container.py
@@ -118,6 +118,6 @@ class TestAssignContainer(test.bootstrap.IFC4):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         aggregate = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcElementAssembly")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=aggregate)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=aggregate)
         ifcopenshell.api.run("spatial.assign_container", self.file, products=[subelement], relating_structure=element)
         assert not ifcopenshell.util.element.get_aggregate(subelement)
diff --git a/src/ifcopenshell-python/test/api/test_api.py b/src/ifcopenshell-python/test/api/test_api.py
index 0a2cd611de..9695849875 100644
--- a/src/ifcopenshell-python/test/api/test_api.py
+++ b/src/ifcopenshell-python/test/api/test_api.py
@@ -83,3 +83,11 @@ class TestTemporarySupportForDeprecatedAPIArguments(test.bootstrap.IFC4):
         ifcopenshell.api.run("layer.unassign_layer", self.file, item=items[2], layer=layer)
         assert len(layer.AssignedItems) == 2
         assert set(layer.AssignedItems) == set(items[:2])
+
+    @deprecation_check
+    def test_assigning_an_aggregate(self):
+        element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcSite")
+        subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
+        rel = ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        assert ifcopenshell.util.element.get_aggregate(subelement) == element
+        assert rel.is_a("IfcRelAggregates")
diff --git a/src/ifcopenshell-python/test/util/test_element.py b/src/ifcopenshell-python/test/util/test_element.py
index 6e70c64473..7b5c674da5 100644
--- a/src/ifcopenshell-python/test/util/test_element.py
+++ b/src/ifcopenshell-python/test/util/test_element.py
@@ -706,7 +706,7 @@ class TestGetContainerIFC4(test.bootstrap.IFC4):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcElementAssembly")
         building = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         ifcopenshell.api.run("spatial.assign_container", self.file, products=[element], relating_structure=building)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         assert subject.get_container(subelement) == building
 
     def test_getting_nothing_if_we_enforce_only_getting_direct_spatial_containers(self):
@@ -714,7 +714,7 @@ class TestGetContainerIFC4(test.bootstrap.IFC4):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcElementAssembly")
         building = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         ifcopenshell.api.run("spatial.assign_container", self.file, products=[element], relating_structure=building)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         assert subject.get_container(subelement, should_get_direct=True) is None
 
 
@@ -736,7 +736,7 @@ class TestGetDecompositionIFC4(test.bootstrap.IFC4):
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBeam")
         building = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         ifcopenshell.api.run("spatial.assign_container", self.file, products=[element], relating_structure=building)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         results = subject.get_decomposition(building)
         assert element in results
         assert subelement in results
@@ -756,7 +756,7 @@ class TestGetAggregateIFC4(test.bootstrap.IFC4):
     def test_getting_the_containing_aggregate_of_a_subelement(self):
         element = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcWall")
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcCovering")
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         assert subject.get_aggregate(subelement) == element
 
 
diff --git a/src/ifcopenshell-python/test/util/test_selector.py b/src/ifcopenshell-python/test/util/test_selector.py
index ac00bab1a8..3053c1a0a6 100644
--- a/src/ifcopenshell-python/test/util/test_selector.py
+++ b/src/ifcopenshell-python/test/util/test_selector.py
@@ -236,9 +236,9 @@ class TestFilterElements(test.bootstrap.IFC4):
         project = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcProject", name="Project")
         ifcopenshell.api.run("spatial.assign_container", self.file, products=[element], relating_structure=space)
         ifcopenshell.api.run("spatial.assign_container", self.file, products=[element2], relating_structure=storey)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=space, relating_object=storey)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=storey, relating_object=building)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=building, relating_object=project)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[space], relating_object=storey)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[storey], relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[building], relating_object=project)
         assert subject.filter_elements(self.file, "IfcWall, location=NULL") == set()
         assert subject.filter_elements(self.file, "IfcWall, location=Space") == {element}
         assert subject.filter_elements(self.file, "IfcWall, location=G") == {element, element2}
@@ -469,7 +469,7 @@ class TestSelector(test.bootstrap.IFC4):
         subelement = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcMember")
         building = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcBuilding")
         ifcopenshell.api.run("spatial.assign_container", self.file, products=[element], relating_structure=building)
-        ifcopenshell.api.run("aggregate.assign_object", self.file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", self.file, products=[subelement], relating_object=element)
         assert set(subject.Selector.parse(self.file, "@ .IfcBuilding")) == {element, subelement}
 
     def test_selecting_elements_from_a_prefiltered_list(self):
diff --git a/src/ifcpatch/test/test_ExtractElements.py b/src/ifcpatch/test/test_ExtractElements.py
index 79ad7e11da..a6b239ee1a 100644
--- a/src/ifcpatch/test/test_ExtractElements.py
+++ b/src/ifcpatch/test/test_ExtractElements.py
@@ -42,8 +42,8 @@ class TestExtractElements:
         building = ifcopenshell.api.run("root.create_entity", ifc_file, ifc_class="IfcBuilding")
         storey = ifcopenshell.api.run("root.create_entity", ifc_file, ifc_class="IfcBuildingStorey")
         wall = ifcopenshell.api.run("root.create_entity", ifc_file, ifc_class="IfcWall")
-        ifcopenshell.api.run("aggregate.assign_object", ifc_file, product=building, relating_object=site)
-        ifcopenshell.api.run("aggregate.assign_object", ifc_file, product=storey, relating_object=building)
+        ifcopenshell.api.run("aggregate.assign_object", ifc_file, products=[building], relating_object=site)
+        ifcopenshell.api.run("aggregate.assign_object", ifc_file, products=[storey], relating_object=building)
         ifcopenshell.api.run("spatial.assign_container", ifc_file, products=[wall], relating_structure=storey)
 
         output = ifcpatch.execute({"file": ifc_file, "recipe": "ExtractElements", "arguments": ["IfcWall"]})
@@ -61,7 +61,7 @@ class TestExtractElements:
         container = ifcopenshell.api.run("root.create_entity", ifc_file, ifc_class="IfcBuildingStorey")
         subelement = ifcopenshell.api.run("root.create_entity", ifc_file, ifc_class="IfcWall")
         ifcopenshell.api.run("spatial.assign_container", ifc_file, products=[element], relating_structure=container)
-        ifcopenshell.api.run("aggregate.assign_object", ifc_file, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", ifc_file, products=[subelement], relating_object=element)
 
         output = ifcpatch.execute({"file": ifc_file, "recipe": "ExtractElements", "arguments": ["IfcWall"]})
 
diff --git a/src/ifcsverchok/__init__.py b/src/ifcsverchok/__init__.py
index 7401a90ce9..66a9563c29 100644
--- a/src/ifcsverchok/__init__.py
+++ b/src/ifcsverchok/__init__.py
@@ -179,7 +179,7 @@ class IFC_Sv_write_file(bpy.types.Operator):
             if not (spatial.is_a("IfcSite") or spatial.is_a("IfcBuilding")) and (spatial not in elements_in_buildings):
                 elements = ifcopenshell.util.element.get_decomposition(spatial)
                 ifcopenshell.api.run(
-                    "aggregate.assign_object", file, product=spatial, relating_object=file.by_type("IfcBuilding")[0]
+                    "aggregate.assign_object", file, products=[spatial], relating_object=file.by_type("IfcBuilding")[0]
                 )
 
         for building in file.by_type("IfcBuilding"):
@@ -202,7 +202,7 @@ class IFC_Sv_write_file(bpy.types.Operator):
                 if len(file.by_type("IfcSite")) == 0:
                     ifcopenshell.api.run("root.create_entity", file, ifc_class="IfcSite", name="My Site")
                 ifcopenshell.api.run(
-                    "aggregate.assign_object", file, product=building, relating_object=file.by_type("IfcSite")[0]
+                    "aggregate.assign_object", file, products=[building], relating_object=file.by_type("IfcSite")[0]
                 )
                 try:
                     if file.by_type("IfcSite")[0].Decomposes[0].RelatingObject.is_a("IfcProject"):
@@ -212,7 +212,7 @@ class IFC_Sv_write_file(bpy.types.Operator):
                 ifcopenshell.api.run(
                     "aggregate.assign_object",
                     file,
-                    product=file.by_type("IfcSite")[0],
+                    products=[file.by_type("IfcSite")[0]],
                     relating_object=file.by_type("IfcProject")[0],
                 )
         self.file = file
diff --git a/src/ifcsverchok/nodes/ifc/add_spatial_element.py b/src/ifcsverchok/nodes/ifc/add_spatial_element.py
index ac94ea1a3c..bf9e4a6dee 100644
--- a/src/ifcsverchok/nodes/ifc/add_spatial_element.py
+++ b/src/ifcsverchok/nodes/ifc/add_spatial_element.py
@@ -135,7 +135,7 @@ class SvIfcAddSpatialElement(bpy.types.Node, SverchCustomTreeNode, ifcsverchok.h
                     ifcopenshell.api.run(
                         "aggregate.assign_object",
                         self.file,
-                        product=items,
+                        products=[items],
                         relating_object=result,
                     )
                 else:
@@ -188,7 +188,7 @@ class SvIfcAddSpatialElement(bpy.types.Node, SverchCustomTreeNode, ifcsverchok.h
                             ifcopenshell.api.run(
                                 "aggregate.assign_object",
                                 self.file,
-                                product=added_element,
+                                products=[added_element],
                                 relating_object=result,
                             )
                         else:
diff --git a/src/ifcsverchok/nodes/ifc/write_file.py b/src/ifcsverchok/nodes/ifc/write_file.py
index ba1baae121..b9f03c6e1a 100644
--- a/src/ifcsverchok/nodes/ifc/write_file.py
+++ b/src/ifcsverchok/nodes/ifc/write_file.py
@@ -93,7 +93,7 @@ class SvIfcWriteFile(bpy.types.Node, SverchCustomTreeNode, ifcsverchok.helper.Sv
                 ifcopenshell.api.run(
                     "aggregate.assign_object",
                     file,
-                    product=spatial,
+                    products=[spatial],
                     relating_object=file.by_type("IfcBuilding")[0],
                 )
 
@@ -120,7 +120,7 @@ class SvIfcWriteFile(bpy.types.Node, SverchCustomTreeNode, ifcsverchok.helper.Sv
                 ifcopenshell.api.run(
                     "aggregate.assign_object",
                     file,
-                    product=building,
+                    products=[building],
                     relating_object=file.by_type("IfcSite")[0],
                 )
                 try:
@@ -131,7 +131,7 @@ class SvIfcWriteFile(bpy.types.Node, SverchCustomTreeNode, ifcsverchok.helper.Sv
                 ifcopenshell.api.run(
                     "aggregate.assign_object",
                     file,
-                    product=file.by_type("IfcSite")[0],
+                    products=[file.by_type("IfcSite")[0]],
                     relating_object=file.by_type("IfcProject")[0],
                 )
         self.file = file
diff --git a/src/ifctester/test/ids_doc_generator.py b/src/ifctester/test/ids_doc_generator.py
index dd002e33c7..3290104cfa 100644
--- a/src/ifctester/test/ids_doc_generator.py
+++ b/src/ifctester/test/ids_doc_generator.py
@@ -320,9 +320,9 @@ site = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcSite", na
 building = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcBuilding", name="Building A")
 storey = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcBuildingStorey", name="Ground Floor")
 
-ifcopenshell.api.run("aggregate.assign_object", model, relating_object=project, product=site)
-ifcopenshell.api.run("aggregate.assign_object", model, relating_object=site, product=building)
-ifcopenshell.api.run("aggregate.assign_object", model, relating_object=building, product=storey)
+ifcopenshell.api.run("aggregate.assign_object", model, relating_object=project, products=[site])
+ifcopenshell.api.run("aggregate.assign_object", model, relating_object=site, products=[building])
+ifcopenshell.api.run("aggregate.assign_object", model, relating_object=building, products=[storey])
 
 wall_types = []
 for i in range(0, 4):
diff --git a/src/ifctester/test/test_facet.py b/src/ifctester/test/test_facet.py
index ed453af65e..0c93fbdc52 100644
--- a/src/ifctester/test/test_facet.py
+++ b/src/ifctester/test/test_facet.py
@@ -1428,7 +1428,7 @@ class TestPartOf:
         subelement = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcWall")
         facet = PartOf(name="IFCELEMENTASSEMBLY", relation="IFCRELAGGREGATES")
         run("A non aggregated element fails an aggregate relationship", facet=facet, inst=subelement, expected=False)
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[subelement], relating_object=element)
         run("The aggregated whole fails an aggregate relationship", facet=facet, inst=element, expected=False)
         run("The aggregated part passes an aggregate relationship", facet=facet, inst=subelement, expected=True)
 
@@ -1439,7 +1439,7 @@ class TestPartOf:
         ifc = ifcopenshell.file()
         element = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcSlab")
         subelement = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcBeam")
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[subelement], relating_object=element)
         facet = PartOf(name="IFCSLAB", relation="IFCRELAGGREGATES")
         run("An aggregate may specify the entity of the whole 1/2", facet=facet, inst=subelement, expected=True)
         facet = PartOf(name="IFCWALL", relation="IFCRELAGGREGATES")
@@ -1462,8 +1462,8 @@ class TestPartOf:
         element = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcElementAssembly")
         subelement = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcSlab")
         subsubelement = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcBeam")
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=subelement, relating_object=element)
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=subsubelement, relating_object=subelement)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[subelement], relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[subsubelement], relating_object=subelement)
         facet = PartOf(name="IFCELEMENTASSEMBLY", relation="IFCRELAGGREGATES")
         run("An aggregate entity may pass any ancestral whole passes", facet=facet, inst=subsubelement, expected=True)
 
@@ -1517,7 +1517,7 @@ class TestPartOf:
         ifc = ifcopenshell.file()
         element = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcSlab")
         subelement = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcBeam")
-        ifcopenshell.api.run("aggregate.assign_object", ifc, product=subelement, relating_object=element)
+        ifcopenshell.api.run("aggregate.assign_object", ifc, products=[subelement], relating_object=element)
         container = ifcopenshell.api.run("root.create_entity", ifc, ifc_class="IfcSpace")
         ifcopenshell.api.run("spatial.assign_container", ifc, products=[element], relating_structure=container)
         facet = PartOf(relation="IFCRELCONTAINEDINSPATIALSTRUCTURE", name="IFCSPACE")