diff --git a/odoo/addons/test_new_api/tests/test_new_fields.py b/odoo/addons/test_new_api/tests/test_new_fields.py
index 0195277d990..3cb4c3b2d78 100644
--- a/odoo/addons/test_new_api/tests/test_new_fields.py
+++ b/odoo/addons/test_new_api/tests/test_new_fields.py
@@ -1654,6 +1654,16 @@ class TestFields(TransactionCaseWithUserDemo):
         self.assertNotEqual(new_disc.participants, disc.participants)
         self.assertEqual(new_disc.participants._origin, disc.participants)
 
+        # check convert_to_write
+        tag = self.env['test_new_api.multi.tag'].create({'name': 'Foo'})
+        rec = self.env['test_new_api.multi'].create({
+            'lines': [(0, 0, {'tags': [(6, 0, tag.ids)]})],
+        })
+        new = rec.new(origin=rec)
+        self.assertEqual(new.lines.tags._origin, rec.lines.tags)
+        vals = new._convert_to_write(new._cache)
+        self.assertEqual(vals['lines'], [(6, 0, rec.lines.ids)])
+
     def test_41_new_compute(self):
         """ Check recomputation of fields on new records. """
         move = self.env['test_new_api.move'].create({
diff --git a/odoo/fields.py b/odoo/fields.py
index 4a7d8464a09..b46c2b0e97c 100644
--- a/odoo/fields.py
+++ b/odoo/fields.py
@@ -3107,6 +3107,9 @@ class _RelationalMulti(_Relational):
             value = record.env[self.comodel_name].browse(value)
 
         if isinstance(value, BaseModel) and value._name == self.comodel_name:
+            def get_origin(val):
+                return val._origin if isinstance(val, BaseModel) else val
+
             # make result with new and existing records
             inv_names = {field.name for field in record._field_inverses[self]}
             result = [Command.set([])]
@@ -3125,7 +3128,7 @@ class _RelationalMulti(_Relational):
                         values = record._convert_to_write({
                             name: record[name]
                             for name in record._cache
-                            if name not in inv_names and record[name] != origin[name]
+                            if name not in inv_names and get_origin(record[name]) != origin[name]
                         })
                         if values:
                             result.append(Command.update(origin.id, values))