[FIX] mail: compute resource_ref

Before this, the resource_ref field is computed once in the default_get.
This means if the cache is invalidated, its value is unrecoverable.

Notably, the cache is invalidated when an attachment is generated
(triggering a commit).

So calling _generate_template could invalidate resource_ref,
which we use as a key to retrieve its result.

This results in a traceback for the user when trying to preview
a template where the first record hasn't yet generated its attachment.

As an example, trying to preview `Sales: Send Quotation`
on fresh installs.

task-3162320

X-original-commit: b4bd93c9b61736a7f41d70afe5db977a6e4b35e0
Part-of: odoo/odoo#118710

addons/mail/wizard/mail_template_preview.py

@@ -28,21 +28,16 @@ class MailTemplatePreview(models.TransientModel):
     def _selection_languages(self):
         return self.env['res.lang'].get_installed()
 
-    @api.model
-    def default_get(self, fields):
-        result = super(MailTemplatePreview, self).default_get(fields)
-        if not result.get('mail_template_id') or 'resource_ref' not in fields:
-            return result
-        mail_template = self.env['mail.template'].browse(result['mail_template_id']).sudo()
-        model = mail_template.model
-        res = self.env[model].search([], limit=1)
-        if res:
-            result['resource_ref'] = '%s,%s' % (model, res.id)
-        return result
 
     mail_template_id = fields.Many2one('mail.template', string='Related Mail Template', required=True)
     model_id = fields.Many2one('ir.model', string='Targeted model', related="mail_template_id.model_id")
-    resource_ref = fields.Reference(string='Record', selection='_selection_target_model')
+    resource_ref = fields.Reference(
+        string='Record',
+        compute='_compute_resource_ref',
+        compute_sudo=False, readonly=False,
+        selection='_selection_target_model',
+        store=True
+    )
     lang = fields.Selection(_selection_languages, string='Template Preview Language')
     no_record = fields.Boolean('No Record', compute='_compute_no_record')
     error_msg = fields.Char('Error Message', compute='_compute_mail_template_fields')
@@ -88,6 +83,14 @@ class MailTemplatePreview(models.TransientModel):
                 error_msg = user_error.args[0]
         self.error_msg = error_msg
 
+    @api.depends('mail_template_id')
+    def _compute_resource_ref(self):
+        for preview in self:
+            mail_template = preview.mail_template_id.sudo()
+            model = mail_template.model
+            res = self.env[model].search([], limit=1)
+            preview.resource_ref = f'{model},{res.id}' if res else False
+
     def _set_mail_attributes(self, values=None):
         for field in self._MAIL_TEMPLATE_FIELDS:
             if field in ('partner_to', 'report_template_ids'):

addons/test_mail/tests/test_mail_template_preview.py

@@ -34,6 +34,31 @@ class TestMailTemplateTools(TestMailTemplateCommon):
                 continue
             self.assertEqual(test_template[field], preview[field])
 
+    def test_mail_template_preview_dynamic_attachment(self):
+        """Check behaviour with templates that use reports."""
+        test_record = self.env['mail.test.lang'].browse(self.test_record.ids)
+        test_report = self.env['ir.actions.report'].sudo().create({
+                'name': 'Test Report',
+                'model': test_record._name,
+                'print_report_name': "'TestReport for %s' % object.name",
+                'report_type': 'qweb-pdf',
+                'report_name': 'test_mail.mail_test_ticket_test_template',
+        })
+        self.test_template.write({
+            'report_template_ids': test_report.ids,
+            'attachment_ids': False,
+        })
+
+        preview = self.env['mail.template.preview'].with_context({
+            'force_report_rendering': False, # this also invalidates the test records...
+            }).create({
+            'mail_template_id': self.test_template.id,
+            'resource_ref': test_record,
+        })
+
+        self.assertEqual(preview.body_html, f'<p>EnglishBody for {test_record.name}</p>')
+        self.assertFalse(preview.attachment_ids, 'Reports should not be listed in attachments')
+
     def test_mail_template_preview_force_lang(self):
         test_record = self.env['mail.test.lang'].browse(self.test_record.ids)
         test_record.write({