feat: tag-based product alternatives

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/Models/Product.php

@@ -3,6 +3,7 @@
 namespace App\Models;
 
 use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Collection;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
@@ -173,6 +174,31 @@ class Product extends Model
         return $this->belongsToMany(Tag::class);
     }
 
+    /**
+     * Get approved products sharing tags with this one, best matches first.
+     * Single query; each result has a shared_tags_count attribute.
+     */
+    public function alternatives(int $limit = 3): Collection
+    {
+        return static::query()
+            ->approved()
+            ->where('products.id', '!=', $this->id)
+            ->join('product_tag', 'product_tag.product_id', '=', 'products.id')
+            ->whereIn('product_tag.tag_id', function ($query) {
+                $query->select('tag_id')->from('product_tag')->where('product_id', $this->id);
+            })
+            ->select('products.*')
+            ->selectRaw('COUNT(product_tag.tag_id) as shared_tags_count')
+            ->withCount('upvotes')
+            ->groupBy('products.id')
+            ->orderByDesc('shared_tags_count')
+            ->orderByDesc('upvotes_count')
+            ->orderByDesc('products.created_at')
+            ->orderByDesc('products.id')
+            ->limit($limit)
+            ->get();
+    }
+
     /**
      * Scope a query to only include approved products.
      */