找不到Laravel Seeder

创建表后，在迁移期间我正在运行一些播种机。这是我的迁移文件create_institutions_table

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

class CreateInstitutionsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('institutions', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->string('name');
            $table->string('code');
            $table->timestamps();
            $table->softDeletes();
        });

        $seeder = new InstitutionsSeeder();
        $seeder->run();

        $seeder2 = new UsersSeeder();
        $seeder2->run();

        Schema::table('users', function (Blueprint $table) {
            $table->foreign('institution_id')->references('id')->on('institutions');
        });
    }

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

这是 InstitutionsSeeder

use Illuminate\Database\Seeder;

class InstitutionsSeeder extends Seeder
{
    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        DB::table('institutions')->insert([
            'name' => 'Institution One',
            'code' => 'I1',
        ]);

    }
}

这是 UsersSeeder

use Illuminate\Database\Seeder;

class UsersSeeder extends Seeder
{
    /**
     * Run the database seeds.
     *
     * @return void
     */
    public function run()
    {
        DB::table('users')->insert([
            'first_name' => 'Admin',
            'last_name' => 'Istrator',
            'email' => '[email protected]',
            'institution_id' => '1',
            'password' => '$2y$10$/wYQaaaaaaagrtyh64gbdt4yuhr32l4VmFHI.sINMR/9LXsj1MTy',
        ]);
    }
}

据我所知，播种机之间没有真正的区别，但是当尝试UsersSeeder在InstitutionsSeeder正常工作的情况下实例化类时，迁移失败。这是我从php artisan migrate:fresh命令中得到的例外：

   Symfony\Component\Debug\Exception\FatalThrowableError  : Class 'UsersSeeder' not found

  at H:\code\MyProject\database\migrations\2019_06_17_224612_create_institutions_table.php:27
    23| 
    24|                 $seeder = new InstitutionsSeeder();
    25|                 $seeder->run();
    26| 
  > 27|                 $seeder2 = new UsersSeeder();
    28|                 $seeder2->run();
    29| 
    30|                 Schema::table('users', function (Blueprint $table) {
    31|                         $table->foreign('institution_id')->references('id')->on('institutions');

  Exception trace:

  1   CreateInstitutionsTable::up()
      H:\code\MyProject\vendor\laravel\framework\src\Illuminate\Database\Migrations\Migrator.php:379

  2   Illuminate\Database\Migrations\Migrator::Illuminate\Database\Migrations\{closure}()
      H:\code\MyProject\vendor\laravel\framework\src\Illuminate\Database\Migrations\Migrator.php:388

  Please use the argument -v to see more details.

为什么不UsersSeeder工作？