Laravel 5.2，友谊模型，如何在用户模型中正确添加？

public function friends()
{
    return $this->belongsToMany(User::class, 'friendships', 'user_id', 'friend_id');
}

php artisan make:migration create_friendships_table --create=friendships

public function up()
{
    Schema::create('friendships', function (Blueprint $table) {
        $table->unsignedBigInteger('user_id');
        $table->unsignedBigInteger('friend_id');
        $table->timestamps();

        $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
        $table->foreign('friend_id')->references('id')->on('users')->onDelete('cascade');
    });
}

php artisan migrate

$user = User::find(1);
$friends = $user->friends;