phpvms/app/Database/factories/SubfleetFactory.php

<?php

use Faker\Generator as Faker;

$factory->define(App\Models\Subfleet::class, function (Faker $faker) {
    return [
        'id' => null,
        'airline_id' => function () {
            return factory(App\Models\Airline::class)->create()->id;
        },
        'name' => $faker->unique()->text(50),
        'type' => $faker->unique()->text(7),
    ];
});