laravel,迁移,外键约束格式不正确

我有两张桌子。产品和机构。有一个主键 pid 哪个自动递增,还有一个 id 哪个是 varchar 属于 32 机构和产品的长度。

每个 product 需要参考其 institution 因此,我设置 机构 列在 products 并希望它是引用 身份证件 机构的。

然而,我一直在犯这个错误,我花了一整晚的时间试图找出这个问题,但没有成功。

$ php artisan migrate
Migration table created successfully.

   Illuminate\Database\QueryException  : SQLSTATE[HY000]: General error: 1005 Can't create table `devbase`.`#sql-824_f23` (errno: 150 "Foreign key constraint is incorrectly formed") (SQL: alter table `institutions` add constraint `institutions_id_foreign` foreign key (`id`) references `institution` (`products`) on delete RESTRICT)

  at [...]\vendor\laravel\framework\src\Illuminate\Database\Connection.php:664
    660|         // If an exception occurs when attempting to run a query, we'll format the error
    661|         // message to include the bindings with SQL, which will make this exception a
    662|         // lot more helpful to the developer instead of just the database's errors.
    663|         catch (Exception $e) {
  > 664|             throw new QueryException(
    665|                 $query, $this->prepareBindings($bindings), $e
    666|             );
    667|         }
    668|

  Exception trace:

  1   PDOException::("SQLSTATE[HY000]: General error: 1005 Can't create table `devbase`.`#sql-824_f23` (errno: 150 "Foreign key constraint is incorrectly formed")")
      [...]\vendor\laravel\framework\src\Illuminate\Database\Connection.php:458

  2   PDOStatement::execute()
      [...]\vendor\laravel\framework\src\Illuminate\Database\Connection.php:458

  Please use the argument -v to see more details.

这是migration for institutions表,文件名是 2018_06_14_165449_create_institutions_table.php

public function up()
{
    Schema::create('institutions', function (Blueprint $table) {
        $table->increments('pid');
        $table->string('id', 32)->unique();
        $table->string('shortname', 16)->unique();
        $table->text('fullname');
        $table->string('logo', 100);
        $table->timestamps();
    });
}

而这是products表的迁移,文件名是 2018_06_15_031837_create_products_table.php

public function up()
{
    Schema::enableForeignKeyConstraints();
    Schema::create('products', function (Blueprint $table) {
        $table->increments('pid');
        $table->string('id', 32)->unique();
        $table->string('url', 100)->unique();
        $table->string('shortname', 16)->unique();
        $table->string('institution', 32)->index()->nullable();
        $table->timestamps();

        $table->foreign('institution')
            ->references('institutions')
            ->on('id')
            ->onDelete('RESTRICT');
    });
}