exchange/database/migrations/2018_06_21_040652_create_invoice_table.php

<?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class CreateInvoiceTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('invoices', function (Blueprint $table) {
            $table->increments('id');
            $table->string('invoice_url');
            $table->double('amount');
            $table->integer('booking_id')->unsigned();
            $table->foreign('booking_id')->references('id')->on('bookings');
            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('invoices');
    }
}