exchange/app/Invoice.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Invoice extends Model
{
    protected $fillable = ['invoice_url','amount','booking_id', 'buyer_company', 'address', 'contact_no', 'po_number'];

    public function booking(){
        return $this->belongsTo('App\Booking');
    }
}