[FIX] website_forum: avoid search in a loop for last post

Computation of the last post shouldn't be done with a
db request for each forum but can be batched.

Technical note:
As it is likely that `post_ids` is not yet fetched when we're
accessing `last_post_id` we chose not to early return if no
forum in `self` included posts already.

Version choice:
We chose this version to merge this as it is the one where
forum was taken over, and _read_group refactored.

A test for this computation is also added.

Task-3685002

closes odoo/odoo#150444

X-original-commit: ba3af942deee4818edb32d08b9bb07652253a029
Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>
Signed-off-by: Florian Charlier (flch) <flch@odoo.com>

addons/website_forum/models/forum_forum.py

@@ -198,11 +198,13 @@ class Forum(models.Model):
 
     @api.depends('post_ids')
     def _compute_last_post_id(self):
+        last_forums_posts = self.env['forum.post']._read_group(
+            [('forum_id', 'in', self.ids), ('parent_id', '=', False), ('state', '=', 'active')],
+            groupby=['forum_id'], aggregates=['id:max'],
+        )
+        forum_to_last_post_id = {forum.id: last_post_id for forum, last_post_id in last_forums_posts}
         for forum in self:
-            forum.last_post_id = forum.post_ids.search(
-                [('forum_id', '=', forum.id), ('parent_id', '=', False), ('state', '=', 'active')],
-                order='create_date desc', limit=1,
-            )
+            forum.last_post_id = forum_to_last_post_id.get(forum.id, False)
 
     @api.depends('post_ids.state', 'post_ids.views', 'post_ids.child_count', 'post_ids.favourite_count')
     def _compute_forum_statistics(self):

addons/website_forum/tests/test_forum_internals.py

@@ -59,6 +59,24 @@ class TestForumInternals(TestForumCommon):
         self.assertEqual(website_2.forum_count, 6,
                          '6 global forums')
 
+    def test_website_forum_last_post_id(self):
+        """Check that each forum's last post is computed correctly and efficiently."""
+        test_forums = self.base_forum | self.forum
+        new_posts = self.env["forum.post"].create([{
+            'name': f'New Post {forum_post_idx}',
+            'forum_id': forum.id,
+        } for forum_post_idx, forum in enumerate(test_forums)])
+
+        with self.assertQueryCount(1):
+            self.assertEqual(test_forums.last_post_id.ids, new_posts.ids)
+
+        another_post = self.env["forum.post"].create([{
+            'name': 'Another New Post',
+            'forum_id': self.base_forum.id,
+        }])
+        with self.assertQueryCount(1):
+            self.assertEqual(test_forums.last_post_id.ids, (another_post | new_posts[1]).ids)
+
 
 @tagged('forum_internals')
 class TestPostInternals(TestForumCommon):