finance_app/app/Models/Expense.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Expense extends Model
{
    protected $fillable = [
        'user_id',
        'category_id',
        'subcategory_id',
        'type',
        'amount',
        'note',
        'date',
        'invoice_number',
        'seller_name',
        'item_name',
        'external_id',
    ];

    protected $casts = [
        'date'   => 'date:Y-m-d',
        'amount' => 'decimal:2',
    ];

    public function user()
    {
        return $this->belongsTo(User::class);
    }

    public function category()
    {
        return $this->belongsTo(Category::class);
    }

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