Pivot table returns empty array

I am trying to get data from pivot table(I have a third column). but it returns empty array. When I do dd() I get this

Collection {#323 ▼
  #items: []
}

$a = new Subject();
$authorized_users = $a->users()->get(['subject_id']);

models

Subject

public function users()
{
    return $this->belongsToMany('App\User')->withPivot('auth_teacher')->withTimestamps();
}

User

public function subjects()
{
    return $this->belongsToMany('App\Subject')->withPivot('auth_teacher')->withTimestamps();
}   

via Chebli Mohamed