# UNDF: UNDF-2026-000000897
--- a/trytond/trytond/model/modelstorage.py
+++ b/trytond/trytond/model/modelstorage.py
@@ -2115,7 +2115,7 @@
                 if self.id is not None and self.id >= 0:
                     _values, self._values = self._values, None
                     try:
-                        previous = [t.id for t in getattr(self, fname)]
+                        previous = set(t.id for t in getattr(self, fname))
                     finally:
                         self._values = _values
                 else:
-                    previous = []
+                    previous = set()
                 to_add = []
                 to_create = []
                 to_write = []
@@ -2139,7 +2139,7 @@
                         else:
                             if target.id in previous:
-                                previous.remove(target.id)
+                                previous.discard(target.id)
                             else:
                                 to_add.append(target.id)
                             target_values = target._save_values()