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

src/ifcopenshell-python/ifcopenshell/api/classification/add_reference.py

@@ -0,0 +1,63 @@
+import ifcopenshell
+import ifcopenshell.util.schema
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "product": None,
+            "reference": None,
+            "classification": None,
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        relating_classification = None
+
+        if hasattr(self.settings["reference"], "ItemReference"):
+            identification = self.settings["reference"].ItemReference  # IFC2X3
+        else:
+            identification = self.settings["reference"].Identification
+
+        for reference in self.file.by_type("IfcClassificationReference"):
+            if self.file.schema == "IFC2X3":
+                if reference.ItemReference == identification:
+                    relating_classification = reference
+                    break
+            else:
+                if reference.Identification == identification:
+                    relating_classification = reference
+                    break
+
+        if relating_classification:
+            association = self.get_association(relating_classification)
+            related_objects = set(association.RelatedObjects)
+            related_objects.add(self.settings["product"])
+            association.RelatedObjects = list(related_objects)
+            return
+
+        migrator = ifcopenshell.util.schema.Migrator()
+        # This removal patch is to support a lightweight classification
+        old_referenced_source = self.settings["reference"].ReferencedSource
+        self.settings["reference"].ReferencedSource = None
+        relating_classification = migrator.migrate(self.settings["reference"], self.file)
+        relating_classification.ReferencedSource = self.settings["classification"]
+        self.settings["reference"].ReferencedSource = old_referenced_source
+        self.file.create_entity(
+            "IfcRelAssociatesClassification",
+            **{
+                "GlobalId": ifcopenshell.guid.new(),
+                "RelatedObjects": [self.settings["product"]],
+                "RelatingClassification": relating_classification,
+            }
+        )
+
+    def get_association(self, reference):
+        if self.file.schema == "IFC2X3":
+            for association in self.file.by_type("IfcRelAssociatesClassification"):
+                if association.RelatingClassification == reference:
+                    return association
+        elif reference.ClassificationRefForObjects:
+            return reference.ClassificationRefForObjects[0]