按照教程手打代码提示:gravatar () 未定义，全复制教程代码依旧，去掉_user_info.blade.php 中{{ $user->gravatar ('140') }}页面可正常打开，求助？

控制器代码：

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Http\Requests;
use App\Models\User;

class UsersController extends Controller
{
    public function create()
    {
        return view('users.create');
    }

    public function show(User $user)
    {
        return view('users.show',compact('user'));
    }

}

模型代码

<?php

namespace App\Models;

use Illuminate\Notifications\Notifiable;
use Illuminate\Foundation\Auth\User as Authenticatable;

class User extends Authenticatable
{
    use Notifiable;

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'name', 'email', 'password',
    ];

    /**
     * The attributes that should be hidden for arrays.
     *
     * @var array
     */
    protected $hidden = [
        'password', 'remember_token',
    ];

    public function gravatar($size = '100')
    {
        $hash = md5(strtolower(trim($this->attributes['email'])));
        return "http://www.gravatar.com/avatar/$hash?s=$size";
    }
}

模板代码（resources/views/shared/_user_info.blade.php）：

<a href="{{ route('users.show', $user->id) }}">
  <img src="{{ $user->gravatar('140') }}" alt="{{ $user->name }}" class="gravatar"/>
</a>
<h1>{{ $user->name }}</h1>

报错提示：