在 Laravel 中添加字符串类型的外键时出错

Question

我有两张桌子，一张优惠券和一张订单。凭证需要有一个 16 字符的字符串作为 ID，它是随机创建的。当 ID 作为整数递增时，一切正常，但是当更改为字符串时，我无法让它添加外键，出现此错误：

 [Illuminate\Database\QueryException]
 SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint (SQL
 : alter table `orders` add constraint `orders_voucher_id_foreign` foreign k
 ey (`voucher_id`) references `vouchers` (`id`))

 [PDOException]
 SQLSTATE[HY000]: General error: 1215 Cannot add foreign key constraint

这是我的桌子：

    Schema::create('vouchers', function (Blueprint $table) {
        $table->engine = 'InnoDB';
        $table->string('id');
        $table->integer('value');
        $table->string('status');
        $table->timestamps();
    });

    Schema::create('orders', function (Blueprint $table) {
        $table->engine = 'InnoDB';
        $table->increments('id');
        $table->integer('user_id')->unsigned();
        $table->integer('event_id')->unsigned();
        $table->string('voucher_id');
        $table->timestamps();      

        $table->foreign('user_id')->references('id')->on('users');
        $table->foreign('event_id')->references('id')->on('events');
        $table->foreign('voucher_id')->references('id')->on('vouchers');
    });

非常感谢任何反馈。提前致谢。

Answer 1

尝试：

$table->string('id')->unique();