Feature: Recommendation Service

This commit introduces a RecommendationService that is used when indexing products.

The service is utilizing a recommendation rules interface, so many rules can be created and applied during indexing the products

src/Catalog/Services/ProductIndexer.php

@@ -44,6 +44,21 @@ use Spatie\MediaLibrary\MediaCollections\Models\Media;
  *     Reflects stock as of the last reindex only — nothing currently reindexes a
  *     product when an order decrements its stock (see docs/product-listing.md).
  *
+ *   - recommendations (recommendations.id filterable): [{id, name, price, image}, ...]
+ *     up to 4 other products to show alongside this one (a "related products"
+ *     section), sourced from Modules\Core\Catalog\Services\RecommendationService's
+ *     configured rule chain (config('catalog.recommendation_rules')). Embedded
+ *     card data, not just ids, same reasoning as `collections`: renders
+ *     directly with zero extra Meilisearch calls. `name` is resolved via
+ *     translateAttribute() at index time (not through ProductService's
+ *     per-request locale resolution, since indexing has no "current locale"
+ *     the way a storefront request does) — same known index-time-locale
+ *     tradeoff `collections` already has. `recommendations.id` is filterable
+ *     specifically so Modules\Core\Catalog\Listeners\
+ *     ReindexProductsRecommendingProduct can find every product currently
+ *     recommending a given one, when that one changes — there's no Postgres
+ *     relation for this, a recommendation only exists inside the index.
+ *
  * A review is created/edited independently of its product (Modules\Core\Providers\
  * ReviewServiceProvider re-indexes the product on review create/update/delete), so
  * this data doesn't go stale between full reindexes.
@@ -67,6 +82,7 @@ class ProductIndexer extends BaseProductIndexer
             'slugs',
             'channel_ids',
             'in_stock',
+            'recommendations.id',
         ];
     }
 
@@ -126,6 +142,16 @@ class ProductIndexer extends BaseProductIndexer
         $data['in_stock'] = $model->variants->contains(
             fn (ProductVariant $variant) => $variant->canBeFulfilledAtQuantity(1)
         );
+        $data['recommendations'] = app(RecommendationService::class)
+            ->recommend($model)
+            ->load(['media', 'variants.prices'])
+            ->map(fn (Product $recommendation) => [
+                'id' => $recommendation->id,
+                'name' => $recommendation->translateAttribute('name'),
+                'price' => $this->cheapestPrice($recommendation, $currency),
+                'image' => $recommendation->media->first() ? $this->mapMedia($recommendation->media->first())['thumb'] : null,
+            ])
+            ->all();
 
         return $data;
     }

src/Catalog/Services/RecommendationService.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace Modules\Core\Catalog\Services;
+
+use Illuminate\Support\Collection;
+use Lunar\Models\Product;
+use Modules\Core\Catalog\Contracts\RecommendationRule;
+
+/**
+ * Runs each rule in config('catalog.recommendation_rules'), in order,
+ * topping up from each successive rule until $limit distinct products are
+ * collected or every rule is exhausted — e.g. 3 from SameCategoryRule
+ * (the product's category only has 3 other products) + 1 from RandomRule.
+ * No rule is special-cased as "the fallback" here; a store gets fallback
+ * behaviour purely by how it orders its own config (e.g. SameCategoryRule
+ * before RandomRule). Never returns the same product twice even if two
+ * rules would both suggest it (see RecommendationRule's $exclude), and
+ * never returns fewer than $limit unless the store genuinely doesn't have
+ * that many other products at all.
+ */
+class RecommendationService
+{
+    /**
+     * @return Collection<int, Product>
+     */
+    public function recommend(Product $product, int $limit = 4): Collection
+    {
+        $recommendations = collect();
+
+        foreach (config('catalog.recommendation_rules', []) as $ruleClass) {
+            if ($recommendations->count() >= $limit) {
+                break;
+            }
+
+            $exclude = [$product->id, ...$recommendations->pluck('id')];
+            $remaining = $limit - $recommendations->count();
+
+            /** @var RecommendationRule $rule */
+            $rule = app($ruleClass);
+            $recommendations = $recommendations->merge(
+                $rule->recommend($product, $remaining, $exclude)
+            );
+        }
+
+        return $recommendations->take($limit)->values();
+    }
+}