Laravel comments and user relationship returns null - php

Question

I'm trying to implement commenting system on my website but i'm having hard time with the relationships. for some reason when i try to access the user relation from the comment class it returns null.. my code:
Post.php
class Post extends Model
{
public $timestamps = false;
public $primaryKey = 'post_id';
protected $fillable = ['title','created_at','username','image'];
public function user()
{
return $this->belongsTo(User::class,'username');
}
public function votes()
{
//Quick note: Id refers to the post id.
return $this->hasMany(PostVotes::class,'post_id');
}
public function comments()
{
return $this->hasMany(Comment::class,'post_id');
}
}
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

Answer 1

Use missed the return keyword
public function user()
{
return $this->belongsTo(User::class,'username') ;
}
public function post()
{
return $this->belongsTo(Post::class,'post_id');
}

Answer 2

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'));
}