update: add inventory limit

app/Services/PlanService.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Services;
+
+use App\Models\Plan;
+
+class PlanService
+{
+    public $plan;
+
+    public function __construct(int $planId)
+    {
+        $this->plan = Plan::lockForUpdate()->find($planId);
+    }
+
+    public function incrementInventory()
+    {
+        if ($this->plan->inventory_limit !== NULL) {
+            return $this->plan->increment('inventory_limit');
+        }
+    }
+
+    public function decrementInventory()
+    {
+        if ($this->plan->inventory_limit !== NULL) {
+            return $this->plan->decrement('inventory_limit');
+        }
+    }
+}