问如何删除Laravel中的外键约束？

EN

 public function storeStudent(Request $request)
{
       $created_user = User::create([
        'name' => $request->name,
        'email' => $request->email,
        'password' => bcrypt($request->password),
        'parent_id' => $request->parent_id,
        'role_id' => $request->role_id,
        'gender'=> $request->gender,
        'date_of_birth'=> $request->date_of_birth,
        'cnic'=>$request->cnic,
        'religion'=>$request->religion,
        'skills'=>$request->skills,
        'physical'=>$request->physical,
        'emergency_name'=>$request->emergency_name,
        'phone_no'=>$request->phone_no,
        'medical_info'=>$request->medical_info,
        'family_dr'=>$request->family_dr,
        'address'=>$request->address,


    ]);
    Registration::create([

       //reg_id is id for registration table

        'user_id' => $created_user->id,
        'class_id' => $request->class_id,
        'section_id' => $request->section_id,

    ]);

public function destroy(Request $request)
{

    $user = User::findOrFail($request->user_id);
    $user->delete();

    $registration=Registration::findOrFail($request->user_id);
    $registration->delete();
    return back();

}

Schema::create('registrations', function (Blueprint $table) {
    $table->increments('reg_id');
    $table->integer('user_id')->unsigned();
    $table->foreign('user_id')->references('id')->on('users');
    $table->integer('class_id')->unsigned();
    $table->foreign('class_id')->references('id')->on('classses');
    $table->integer('section_id')->unsigned();
    $table->foreign('section_id')->references('id')->on('sections');
    $table->timestamps();
});

 Integrity constraint violation: 1451 Cannot delete or update a parent row: a foreign key constraint fails (`jurs1`.`registrations`, CONSTRAINT `registrations_user_id_foreign` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`)) (SQL: delete from `users` where `id` = 10)