WebAPP/app/Models/EventCarpool.php

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsTo;
use Illuminate\Database\Eloquent\Relations\HasMany;

class EventCarpool extends Model
{
    protected $fillable = ['event_id', 'seats', 'note'];

    protected $casts = [
        'seats' => 'integer',
    ];

    public function event(): BelongsTo
    {
        return $this->belongsTo(Event::class);
    }

    public function driver(): BelongsTo
    {
        return $this->belongsTo(User::class, 'user_id');
    }

    public function passengers(): HasMany
    {
        return $this->hasMany(EventCarpoolPassenger::class, 'carpool_id');
    }

    public function remainingSeats(): int
    {
        return $this->seats - $this->passengers->count();
    }
}