Laravel 5.2 Entrust 迁移错误，无法添加外键约束

在使用 Laravel 5.2 Entrust 迁移时，可能会遇到无法添加外键约束的错误。这个问题可能会导致数据库表之间的关系无法正确建立，进而影响系统的正常运行。在本文中，我们将讨论这个问题的原因，并提供解决方案和案例代码，帮助读者解决类似的错误。

php
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;
class CreateRolesTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('roles', function (Blueprint $table) {
            $table->increments('id');
            $table->string('name')->unique();
            $table->timestamps();
        });
        Schema::create('users', function (Blueprint $table) {
            $table->increments('id');
            $table->string('name');
            $table->string('email')->unique();
            $table->string('password');
            $table->rememberToken();
            $table->timestamps();
        });
        Schema::create('role_user', function (Blueprint $table) {
            $table->integer('role_id')->unsigned();
            $table->integer('user_id')->unsigned();
            $table->foreign('role_id')->references('id')->on('roles')->onDelete('cascade');
            $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
            $table->primary(['role_id', 'user_id']);
        });
    }
    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('role_user');
        Schema::dropIfExists('users');
        Schema::dropIfExists('roles');
    }
}