Category.php

<?php
namespace App\Models;
use Illuminate\Database\Eloquent\Model;

class Category extends Model
{
    protected $with = ["sub_categories"];

    public function sub_categories(){
        return $this->hasMany("App\Models\Category", "parent_id", "id");
    }
    public function parent(){
        return $this->belongsTo("App\Models\Category", "parent_id", "id");
    }
}