[IMP] mail: reset composer mixin fields when removing template

Purpose of this commit is to improve behavior or mail.compose.mixin when
removing the template. It now voids the value to avoid having half baked
definition on composers.

Summary of behavior

  * when choosing a template: existing (non void) values override any value
    in the composer mixin;
  * when removing a template: reset values;
  * when going from templateA to templateB: works like choosing a template
    which means only non void values are taken. This may lead to half-baked
    content but trying to remove old template content based on _origin and
    content comparisons would be complicated for few added value;

This makes the 'mail.compose.mixin' behaves like 'mail.compose.message'
wizard which was recently updated at odoo/odoo#107356.

Task-3093257 (Mail: The Composer Update)

Part-of: odoo/odoo#106177

addons/mail/models/mail_composer_mixin.py

@@ -38,18 +38,24 @@ class MailComposerMixin(models.AbstractModel):
 
     @api.depends('template_id')
     def _compute_subject(self):
+        """ Computation is coming either from template, either reset. When
+        having a template with a value set, copy it. When removing the
+        template, reset it. """
         for composer_mixin in self:
-            if composer_mixin.template_id:
+            if composer_mixin.template_id.subject:
                 composer_mixin.subject = composer_mixin.template_id.subject
-            elif not composer_mixin.subject:
+            elif not composer_mixin.template_id:
                 composer_mixin.subject = False
 
     @api.depends('template_id')
     def _compute_body(self):
+        """ Computation is coming either from template, either reset. When
+        having a template with a value set, copy it. When removing the
+        template, reset it. """
         for composer_mixin in self:
-            if composer_mixin.template_id:
+            if not tools.is_html_empty(composer_mixin.template_id.body_html):
                 composer_mixin.body = composer_mixin.template_id.body_html
-            elif not composer_mixin.body:
+            elif not composer_mixin.template_id:
                 composer_mixin.body = False
 
     @api.depends('body', 'template_id')
@@ -67,8 +73,9 @@ class MailComposerMixin(models.AbstractModel):
 
     @api.depends('template_id')
     def _compute_lang(self):
-        """ Take value form template when set. When removing the template
-        reset the value to avoid keeping part of template configuration. """
+        """ Computation is coming either from template, either reset. When
+        having a template with a value set, copy it. When removing the
+        template, reset it. """
         for composer_mixin in self:
             if composer_mixin.template_id.lang:
                 composer_mixin.lang = composer_mixin.template_id.lang

addons/test_mail/tests/test_mail_composer_mixin.py

@@ -79,24 +79,17 @@ class TestMailComposerMixin(TestMailCommon, TestRecipients):
 
         # template with void values: should not force void (TODO)
         composer.template_id = template_void.id
-        self.assertEqual(composer.body, f'<p><br /></p>',
-                         'TODO: should not force void value')
+        self.assertEqual(composer.body, '<p>CustomBody for <t t-out="object.name"/></p>')
         self.assertFalse(composer.body_has_template_value)
         self.assertEqual(composer.lang, template.lang)
-        self.assertEqual(composer.subject, False,
-                         'TODO: should not force void value')
-        # temporarily reput values
-        composer.body = template.body_html
-        composer.subject = template.subject
+        self.assertEqual(composer.subject, 'CustomSubject for {{ object.name }}')
 
         # reset template TOOD should reset
         composer.write({'template_id': False})
-        self.assertEqual(composer.body, self.mail_template.body_html,
-                         'TODO: should reset')
+        self.assertFalse(composer.body)
         self.assertFalse(composer.body_has_template_value)
         self.assertFalse(composer.lang)
-        self.assertEqual(composer.subject, self.mail_template.subject,
-                         'TODO: should reset')
+        self.assertFalse(composer.subject)
 
     @users("employee")
     def test_rendering_custom(self):