diff --git a/addons/account/tests/test_portal_attachment.py b/addons/account/tests/test_portal_attachment.py
index 2aa1b4eff2c..182c0d33a9b 100644
--- a/addons/account/tests/test_portal_attachment.py
+++ b/addons/account/tests/test_portal_attachment.py
@@ -32,7 +32,7 @@ class TestUi(tests.HttpCase):
         files = [('file', ('test.txt', b'test', 'plain/text'))]
         res = self.url_open(url=create_url, data=create_data, files=files)
         self.assertEqual(res.status_code, 400)
-        self.assertTrue("you do not have the rights" in res.text)
+        self.assertIn("you do not have the rights", res.text)
 
         # Test public user can create attachment with token
         create_data['access_token'] = invoice._portal_ensure_token()
@@ -73,7 +73,7 @@ class TestUi(tests.HttpCase):
         res = self.opener.post(url=remove_url, json={'params': remove_data})
         self.assertEqual(res.status_code, 200)
         self.assertTrue(self.env['ir.attachment'].search([('id', '=', create_res['id'])]))
-        self.assertTrue("you do not have the rights" in res.text)
+        self.assertIn("you do not have the rights", res.text)
 
         # Test attachment can be removed with token if "pending" state
         remove_data['access_token'] = create_res['access_token']
@@ -95,13 +95,14 @@ class TestUi(tests.HttpCase):
         res = self.opener.post(url=remove_url, json={'params': remove_data})
         self.assertEqual(res.status_code, 200)
         self.assertTrue(self.env['ir.attachment'].search([('id', '=', attachment.id)]))
-        self.assertTrue("not in a pending state" in res.text)
+        self.assertIn("not in a pending state", res.text)
 
         # Test attachment can't be removed if attached to a message
         attachment.write({
             'res_model': 'mail.compose.message',
             'res_id': 0,
         })
+        attachment.flush()
         message = self.env['mail.message'].create({
             'attachment_ids': [(6, 0, attachment.ids)],
         })
@@ -112,7 +113,7 @@ class TestUi(tests.HttpCase):
         res = self.opener.post(url=remove_url, json={'params': remove_data})
         self.assertEqual(res.status_code, 200)
         self.assertTrue(attachment.exists())
-        self.assertTrue("it is linked to a message" in res.text)
+        self.assertIn("it is linked to a message", res.text)
         message.unlink()
 
         # Test attachment can't be associated if no attachment token.
diff --git a/odoo/addons/base/models/ir_attachment.py b/odoo/addons/base/models/ir_attachment.py
index d98a52b8ef1..11eacf623fc 100644
--- a/odoo/addons/base/models/ir_attachment.py
+++ b/odoo/addons/base/models/ir_attachment.py
@@ -281,7 +281,8 @@ class IrAttachment(models.Model):
     res_name = fields.Char('Resource Name', compute='_compute_res_name')
     res_model = fields.Char('Resource Model', readonly=True, help="The database object this attachment will be attached to.")
     res_field = fields.Char('Resource Field', readonly=True)
-    res_id = fields.Integer('Resource ID', readonly=True, help="The record id this is attached to.")
+    res_id = fields.Many2oneReference('Resource ID', model_field='res_model',
+                                      readonly=True, help="The record id this is attached to.")
     company_id = fields.Many2one('res.company', string='Company', change_default=True,
                                  default=lambda self: self.env.company)
     type = fields.Selection([('url', 'URL'), ('binary', 'File')],