finance_app/app/Models/Expense.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Expense extends Model
{
    protected $fillable = [
    'user_id',
    'amount',
    'category_id',
    'item_name',
    'date',
    'note',
    'seller_name',
    'account_id',
    'is_amortized',
    'period_start',
    'period_end',
];

    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');
    }

    public function account()
    {
        return $this->belongsTo(Account::class);
    }
}