Add all files

app/Models/ProductUpgrade.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+
+class ProductUpgrade extends Model
+{
+    use HasFactory;
+
+    protected $guarded = [];
+
+    public function product()
+    {
+        return $this->belongsTo(Product::class);
+    }
+
+    public function upgrade()
+    {
+        return $this->belongsTo(Product::class, 'upgrade_id');
+    }
+}