First try this: composer dump-auto (as commented by user1669496)
if this didn't helped then change your model...
Change this:
return $this->belongsTo('Post');
to smth like this:
$this->belongsTo('App\Models\Post');
Do the similar for Post model.
Just change App\Models\XXXX to your namespace where you have Post model saved.
I had similar problem and this helped me, hope it will help you.