Php Laravel注释和用户关系返回null

我试图在我的网站上实现评论系统，但我在处理人际关系方面遇到了困难。出于某种原因，当我试图从comment类访问用户关系时，它返回null。。我的代码： Post.php

User.php

class User extends Model implements Authenticatable
{
  use AuthenticableTrait;
  protected $primaryKey = 'username';
  public $incrementing = false;
  public $timestamps = false;
  protected $fillable = ['username','email','password','created_at','avatar','about'];
  // Gets avatar to display on navbar.
  public function posts()
  {
    return $this->hasMany(Post::class,'username');
  }
  public function comments()
  {
    return $this->hasMany(Comment::class,'username');  
  }
  public static function getAvatar()
  {
     return self::Where('username', '=', Session::get('username'))->value('avatar');
  }

}

Comment.php

    class Comment extends Model
{
    public $timestamps = false;
    public $primaryKey = 'post_id';
    protected $fillable = ['comment','created_at','post_id','username'];
    public function user()
    {
        $this->belongsTo(User::class,'username') ;
    }
    public function post()
    {
        $this->belongsTo(Post::class,'post_id');
    }
}
public function view($post_id)
   {
      $post = Post::with('comments')->findOrFail($post_id);
      return view('posts.view',compact('post'));
   }
  @foreach($post->comments as $comment)
 // null
 {{dd($comment->user())}}
@endforeach

---

使用返回关键字

public function user()
{
    return $this->belongsTo(User::class,'username') ;
}

public function post()
{
    return $this->belongsTo(Post::class,'post_id');
}

---

在user和post函数中，您省略了return关键字。意思是你什么都不回来我的上帝。。。我真不敢相信我会这么蠢，你不会。我们每个人都会遇到这种情况。哈哈，谢谢，但我昨晚熬夜试图解决这个问题30分钟XD

public function user()
{
    return $this->belongsTo(User::class,'username') ;
}

public function post()
{
    return $this->belongsTo(Post::class,'post_id');
}

    public function displaycomment($id) {
        $data['comment'] = comment::select('comments.id', 'comments.description', 'users.name', 'comments.created_at', 'comments.post_id')->where('post_id',$id)->leftjoin('users', 'users.id', '=', 'comments.user_id')->get();
//        dd($data);
        return view('displayblog',  compact('data'));
    }