save relationship of objects

src/knowledge/object/relation.py

@@ -0,0 +1,16 @@
+# define a relation store class
+from .object_id import ObjectID
+
+
+class ObjectRelationStore:
+    def __init__(self, root_dir: str):
+        pass
+
+    def add_relation(self, object: ObjectID, parent: ObjectID):
+        pass
+
+    def get_related_objects(self, object: ObjectID) -> [ObjectID]:
+        pass
+
+    def delete_relation(self, object_id: ObjectID) -> [ObjectID]:
+        pass