finance_app/app/Models/Category.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Category extends Model
{
    protected $fillable = ['user_id', 'parent_id', 'name', 'color', 'icon'];

    public function parent()
    {
        return $this->belongsTo(Category::class, 'parent_id');
    }

    public function children()
    {
        return $this->hasMany(Category::class, 'parent_id');
    }

    public function expenses()
    {
        return $this->hasMany(Expense::class);
    }
}