feat: add a solution filter for the posts endpoint

extend.php

@@ -23,6 +23,7 @@
 use Flarum\Discussion\Filter\DiscussionFilterer;
 use Flarum\Discussion\Search\DiscussionSearcher;
 use Flarum\Extend;
+use Flarum\Post\Filter\PostFilterer;
 use Flarum\Post\Post;
 use Flarum\Settings\Event\Saving as SettingsSaving;
 use Flarum\Tags\Api\Serializer\TagSerializer;
@@ -127,6 +128,9 @@
     (new Extend\Filter(DiscussionFilterer::class))
         ->addFilter(Search\BestAnswerFilterGambit::class),
 
+    (new Extend\Filter(PostFilterer::class))
+        ->addFilter(Search\BestAnswerPostFilter::class),
+
     (new Extend\ApiSerializer(TagSerializer::class))
         ->attributes(function (TagSerializer $serializer, Tag $tag, array $attributes) {
             $attributes['isQnA'] = (bool) $tag->is_qna;

src/Search/BestAnswerPostFilter.php

@@ -0,0 +1,66 @@
+<?php
+
+/*
+ * This file is part of fof/best-answer.
+ *
+ * Copyright (c) FriendsOfFlarum.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace FoF\BestAnswer\Search;
+
+use Flarum\Filter\FilterInterface;
+use Flarum\Filter\FilterState;
+use Flarum\Tags\Tag;
+use Flarum\User\User;
+use Illuminate\Database\Query\Builder;
+use Illuminate\Support\Collection;
+
+class BestAnswerPostFilter implements FilterInterface
+{
+    public function getFilterKey(): string
+    {
+        return 'is:solution';
+    }
+
+    public function filter(FilterState $filterState, string $filterValue, bool $negate)
+    {
+        $this->constrain($filterState->getQuery(), $filterState->getActor(), $negate);
+    }
+
+    protected function constrain(Builder $query, User $actor, bool $negate)
+    {
+        // Join the `discussions` table to access `best_answer_post_id`.
+        $query->join('discussions', 'posts.discussion_id', '=', 'discussions.id')
+            ->where('posts.type', 'comment');
+
+        if ($negate) {
+            // Exclude posts that are marked as the best answer
+            $query->where(function ($query) {
+                $query->whereNull('discussions.best_answer_post_id')
+                    ->orWhereColumn('posts.id', '!=', 'discussions.best_answer_post_id');
+            });
+        } else {
+            // Include only posts that are marked as the best answer
+            $query->whereNotNull('discussions.best_answer_post_id')
+                ->whereColumn('posts.id', '=', 'discussions.best_answer_post_id');
+        }
+
+        // Constrain to discussions within the allowed Q&A tags
+        $query->whereIn('discussions.id', function ($subQuery) use ($actor) {
+            $subQuery->select('discussion_id')
+                ->from('discussion_tag')
+                ->whereIn('tag_id', $this->allowedQnATags($actor));
+        });
+    }
+
+    protected function allowedQnATags(User $actor): Collection
+    {
+        return Tag::query()
+            ->whereVisibleTo($actor)
+            ->where('is_qna', true)
+            ->pluck('id');
+    }
+}