finance_app/app/Models/Account.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Account extends Model
{
    protected $fillable = [
        'user_id',
        'name',
        'type',
        'currency',
        'balance',
        'note',
        'billing_day',
        'payment_day',
    ];

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

    public function holdings()
    {
        return $this->hasMany(InvestmentHolding::class);
    }

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

    public function unpaidAmount()
    {
        $billingDay = $this->billing_day ?? 1;

        $now = now();

        // 如果今天還沒到結帳日，帳單週期是上個月結帳日到這個月結帳日
        if ($now->day < $billingDay) {
            $start = $now->copy()->subMonth()->setDay($billingDay);
            $end = $now->copy()->setDay($billingDay)->subDay();
        } else {
            $start = $now->copy()->setDay($billingDay);
            $end = $now->copy()->addMonth()->setDay($billingDay)->subDay();
        }

        return $this->expenses()
            ->whereBetween('date', [$start->toDateString(), $end->toDateString()])
            ->sum('amount');
    }
}