[FIX] core: first onchange when adding a line in a one2many field

Consider a form view with a one2many field, which has no form subview.
Also the form view of the comodel (the one2many field's lines) contains
the inverse many2one field of the one2many field.  When adding a new
line on some existing record, the form view shows the many2one field as
empty, instead of being the main record.

Explanation: the form view of the line invokes onchange() with the main
record's values (dict) as the value of the many2one field.  Inside
onchange(), the field is actually set to a new record corresponding to
the main record.  Alas, when that value is sent back to the form, the
new record is serialized as False.

Solution: let onchange() serialize the new record as its origin record
instead.

closes odoo/odoo#114648

X-original-commit: d137ea4915da8d46909fcb082b5a88fd347874a4
Signed-off-by: Raphael Collet <rco@odoo.com>

odoo/addons/test_new_api/tests/test_onchange.py

@@ -535,6 +535,20 @@ class TestOnChange(SavepointCaseWithUserDemo):
 
         self.assertFalse(called[0], "discussion.messages has been read")
 
+    def test_onchange_one2many_many2one_in_form(self):
+        order = self.env['test_new_api.monetary_order'].create({
+            'currency_id': self.env.ref('base.USD').id,
+        })
+
+        # this call to onchange() is made when creating a new line in field
+        # order.line_ids; check what happens when the line's form view contains
+        # the inverse many2one field
+        values = {'order_id': {'id': order.id, 'currency_id': order.currency_id.id}}
+        field_onchange = dict.fromkeys(['order_id', 'subtotal'], '')
+        result = self.env['test_new_api.monetary_order_line'].onchange(values, [], field_onchange)
+
+        self.assertEqual(result['value']['order_id'], (order.id, order.display_name))
+
     def test_onchange_inherited(self):
         """ Setting an inherited field should assign the field on the parent record. """
         foo, bar = self.env['test_new_api.multi.tag'].create([{'name': 'Foo'}, {'name': 'Bar'}])

odoo/fields.py

@@ -3019,9 +3019,8 @@ class Many2one(_Relational):
         return value.display_name
 
     def convert_to_onchange(self, value, record, names):
-        if not value.id:
-            return False
-        return super(Many2one, self).convert_to_onchange(value, record, names)
+        # if value is a new record, serialize its origin instead
+        return super().convert_to_onchange(value._origin, record, names)
 
     def write(self, records, value):
         # discard recomputation of self on records