当我运行 php artisan db:seed 时，出现以下错误：

[ReflectionException] Class SongsTableSeeder does not exist

到底是怎么回事？

我的 DatabaseSeeder 类：

<?php

use Illuminate\Database\Seeder;
use Illuminate\Database\Eloquent\Model;

class DatabaseSeeder extends Seeder {

    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        Model::unguard();

        $this->call('SongsTableSeeder');
    }

}

我的 SongsTableSeeder 类：

<?php

// Composer: "fzaninotto/faker": "v1.4.0"
use Faker\Factory as Faker;
use Illuminate\Database\Seeder;
use DB;

class SongsTableSeeder extends Seeder {

    public function run()
    {
        $faker = Faker::create();
        $songs = [];
        foreach(range(1, 10) as $index)
        {
            $songs[] = ['title' => $faker->words(rand(1,4))];
        }

        DB::table('songs')->insert($songs);

    }

}