Laravel 5.1 迁移和 seeder 无法截断外键约束中引用的表

I'm trying to run the migration (see below) and seed the database, but when I run

php artisan migrate --seed

I get this error:

Migration table created successfully.
Migrated: 2015_06_17_100000_create_users_table
Migrated: 2015_06_17_200000_create_password_resets_table
Migrated: 2015_06_17_300000_create_vehicles_table

[Illuminate\Database\QueryException]
SQLSTATE[42000]: Syntax error or access violation: 1701 Cannot truncate a table
referenced in a foreign key constraint (`app`.`vehicles`, CONSTRAINT `vehic
les_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `app`.`users` (`id`
)) (SQL: truncate `users`)

[PDOException]
SQLSTATE[42000]: Syntax error or access violation: 1701 Cannot truncate a table
referenced in a foreign key constraint (`app`.`vehicles`, CONSTRAINT `vehic
les_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `app`.`users` (`id`
))

I looked up what this error is supposed to mean, and also found examples of other people running into the same problem, even just related to using MySQL, and their solutions, but applying:

DB::statement('SET FOREIGN_KEY_CHECKS=0;'); and 
DB::statement('SET FOREIGN_KEY_CHECKS=1;'); 

在down()中似乎不起作用，当我在MySQL中运行descripe时，表看起来是正确的.

迁移的名称正确，以确保先迁移USERS表，然后迁移Vehicles，以便可以应用外键，并且正确设置的表表明迁移已运行，但随后出现错误.我删除并重新创建了数据库，然后再试一次，结果是一样的.我也不明白为什么它试图截断数据库的第一次迁移和种子，当您try 运行php artisan Migration:fresh--Seed时，我想不到会发生这种情况.

// 2015_06_17_100000_create_users_table.php

class CreateUsersTable extends Migration
{
    public function up()
    {
        Schema::create('users', function (Blueprint $table) {
            $table->increments('id');
            $table->string('username', 60)->unique();
            $table->string('email', 200)->unique();
            $table->string('password', 255);
            $table->string('role')->default('user');
            $table->rememberToken();
            $table->timestamps();
        });
    }
}

public function down()
{
    Schema::drop('users');
}

// 2015_06_17_300000_create_vehicles_table.php

class CreateVehiclesTable extends Migration
{
    public function up()
    {
        Schema::create('vehicles', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('user_id')->unsigned();
            $table->string('make');
            $table->string('model');
            $table->string('year');
            $table->string('color');
            $table->string('plate');
            $table->timestamps();

            $table->foreign('user_id')->references('id')->on('users');
        });
    }
}

public function down()
{
    Schema::drop('vehicles');
}