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

src/ifcopenshell-python/ifcopenshell/api/document/assign_document.py

@@ -0,0 +1,41 @@
+import ifcopenshell
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "product": None,
+            "document": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        rel = self.get_document_rel()
+        related_objects = set(rel.RelatedObjects) if rel.RelatedObjects else set()
+        related_objects.add(self.settings["product"])
+        rel.RelatedObjects = list(related_objects)
+
+    def get_document_rel(self):
+        if self.file.schema == "IFC2X3":
+            for rel in self.file.by_type("IfcRelAssociatesDocument"):
+                if rel.RelatingDocument == self.settings["document"]:
+                    return rel
+        else:
+            if (
+                hasattr(self.settings["document"], "DocumentRefForObjects")
+                and self.settings["document"].DocumentRefForObjects
+            ):
+                return self.settings["document"].DocumentRefForObjects[0]
+            elif (
+                hasattr(self.settings["document"], "DocumentInfoForObjects")
+                and self.settings["document"].DocumentInfoForObjects
+            ):
+                return self.settings["document"].DocumentInfoForObjects[0]
+
+        return self.file.create_entity("IfcRelAssociatesDocument", **{
+            "GlobalId": ifcopenshell.guid.new(),
+            # TODO: owner history
+            "RelatingDocument": self.settings["document"]
+        })