从帖子关系中获取用户头像属性

In Laravel 5.8, I have a custom attribute to recover the avatar via Gravatar. This is an attribute in the User model.

/**
 * @return string
 */
public function getAvatarAttribute()
{
    return sprintf('%s%s%s', 'https://secure.gravatar.com/avatar/', md5(strtolower(trim($this->email))), '?s=200');
}

I have a belongsTo/hasMany relationship in the Post/User model.

Post model:

/**
 * @return BelongsTo
 */
public function user(): BelongsTo
{
    return $this->belongsTo(User::class);
}

User model:

<?php

namespace App\Models;

use Illuminate\Database\Eloquent\Relations\HasMany;
use Illuminate\Notifications\Notifiable;
use Illuminate\Contracts\Auth\MustVerifyEmail;
use Illuminate\Foundation\Auth\User as Authenticatable;

class User extends Authenticatable
{
    use Notifiable;

    /**
     * @var string
     */
    protected $table = 'users';

    /**
     * @var array
     */
    protected $fillable = [
        'username',
        'email',
        'password',
        'api_token',
    ];

    /**
     * @var array
     */
    protected $hidden = [
        'password',
        'remember_token',
        'api_token',
    ];

    /**
     * @var array
     */
    protected $casts = [
        'email_verified_at' => 'datetime',
        'admin' => 'boolean',
    ];

    /**
     * @return string
     */
    public function getRouteKeyName()
    {
        return 'username';
    }

    /**
     * @return HasMany
     */
    public function posts(): HasMany
    {
        return $this->hasMany(Post::class);
    }

    /**
     * @return string
     */
    public function getAvatarAttribute()
    {
        return sprintf('%s%s%s', 'https://secure.gravatar.com/avatar/', md5(strtolower(trim($this->email))), '?s=200');
    }

}

I pass the post by the URL of the route:

Route::get('post/{post}', 'BlogController@post');

I would like to retrieve the avatar attribute via post. Only, I recover a null. And I do not understand where it comes from.

public function post(Post $post)
{
    dd($post->user); // user model without appends attributes
    dd($post->user->avatar); // null
}

I found the problem, I used User from Illuminate (Illuminate\Foundation\Auth\User) instead my User model.