Update migrations and add foreign key, update related models

app/Models/Category.php

@@ -264,7 +264,7 @@ class Category extends Model
      */
     public function parent()
     {
-        return $this->belongsTo(static::class, 'parentid');
+        return $this->belongsTo(RootCategory::class, 'root_categories_id');
     }
 
     /**

app/Models/RootCategory.php

@@ -6,5 +6,11 @@ use Illuminate\Database\Eloquent\Model;
 
 class RootCategory extends Model
 {
-    //
+    /**
+     * @return \Illuminate\Database\Eloquent\Relations\HasMany
+     */
+    public function categories()
+    {
+        return $this->hasMany(Category::class, 'root_categories_id');
+    }
 }

database/migrations/2019_03_12_090532_change_categories_table.php

@@ -0,0 +1,33 @@
+<?php
+
+use Illuminate\Support\Facades\Schema;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Database\Migrations\Migration;
+
+class ChangeCategoriesTable extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        Schema::table('categories', function (Blueprint $table) {
+            $table->renameColumn('parentid', 'root_categories_id');
+            $table->foreign(['root_categories_id'], 'fk_root_categories_id')->references('id')->on('root_categories')->onDelete('cascade');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        Schema::table('categories', function (Blueprint $table) {
+            //
+        });
+    }
+}