每个任务只允许一个反馈[Laravel 5]

我想做一个反馈系统，用户只能给每个"任务"项目一个反馈。目前，一个用户可以提供尽可能多的反馈，但我当然想限制这一点，因为它背后有一个评级，等等。

任何帮助将不胜感激!

这些是我的模型:

//Feedback
    public function user()
    {
        return $this->belongsTo('App'User');   
    }
    public function tnb()
    {
        return $this->belongsTo('App'Tnb');   
    }
//Tnb
    public function user()
    {
        return $this->belongsTo('App'User');
    }
    public function feedbacks() 
    {
        return $this->hasMany('App'Feedback');
    }
// User
    public function tnbs()
    {
        return $this->hasMany('App'Tnb');
    }
    public function feedbacks() 
    {   
        return $this->hasMany('App'Feedback');
    }

我的数据库表:

Schema::create('users', function (Blueprint $table) {
            $table->increments('id');
            $table->string('username')->unique();
            $table->string('email')->unique();
            $table->string('password', 60);
            $table->rememberToken();
            $table->timestamps();
        });
        Schema::create('tnb', function (Blueprint $table) {
            $table->increments('id');
            $table->string('type');
            $table->string('name');
            $table->integer('user_id')->unsigned();
            $table->foreign('user_id')->references('id')->on('users')->onCascade('update')->onDelete('cascade');
            $table->integer('group_id')->unsigned();
            $table->foreign('group_id')->references('id')->on('groups')->onCascade('update')->onDelete('cascade');
            $table->string('desc');
            $table->string('slug');
            $table->date('startdate');
            $table->time('starttime');
            $table->date('enddate');
            $table->time('endtime');
            $table->timestamps();
        });
        Schema::create('feedbacks', function (Blueprint $table) {
            $table->increments('id');
            $table->string('score');
            $table->string('feedback');
            $table->integer('user_id')->unsigned();
            $table->foreign('user_id')->references('id')->on('users')->onCascade('update')->onDelete('cascade');
            $table->integer('tnb_id')->unsigned();
            $table->foreign('tnb_id')->references('id')->on('tnb')->onCascade('update')->onDelete('cascade');
            $table->timestamps();
        });

我一直在尝试几件事在我的控制器，但直到现在我没有得到任何地方:

public function store(Request $request, Group $group, Tnb $tnb)
{
    $user = auth()->user();
    $feedback = new Feedback(
        array(
            'score' => $request->get('score'),
            'feedback' => $request->get('feedback')
            ));
    $feedback->user()->associate($user);
    //dd($feedback->user()->whereId(Auth::user()->id)->count());
    $tnb->feedbacks()->save($feedback);
    'Flash::success('Your feedback has succesfully been sumbitted!');
    return redirect()->back();
}