Convert instances in RPC call result for on_change_with on field

Since d142b33fa3ab0fe5af25957f90ee77fe7eee51b3 the on_change_with method can
return instances.

Closes #12217

trytond/trytond/model/fields/field.py

@@ -1,7 +1,7 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 import warnings
-from functools import wraps
+from functools import partial, wraps
 
 import sql
 from sql import (
@@ -195,6 +195,20 @@ def on_change_result(record):
     return record._changed_values
 
 
+def on_change_with_result(field, value):
+    from ..modelstorage import ModelStorage
+    if field._type in {'many2one', 'one2one', 'reference'}:
+        if isinstance(value, ModelStorage):
+            if field._type == 'reference':
+                value = str(value)
+            else:
+                value = value.id
+    elif field._type in {'one2many', 'many2many'}:
+        if isinstance(value, (list, tuple)):
+            value = [int(r) for r in value]
+    return value
+
+
 def domain_method(func):
     @wraps(func)
     def wrapper(self, domain, tables, Model):
@@ -455,7 +469,7 @@ class Field(object):
     def set_rpc(self, model):
         for attribute, result in (
                 ('on_change', on_change_result),
-                ('on_change_with', None),
+                ('on_change_with', partial(on_change_with_result, self)),
                 ):
             if not getattr(self, attribute):
                 continue