Laravel : Auth::user() returns null

I want to do Auth::user() but before that I must do Auth::attempt() but it always returns false because the attempt() method uses ID to look for the user I am are trying to login. But in my users table I don't have a ID field rather I have user_id field. So is there a way to make the attempt() method use user_id to look up for id in my users table. (Hope I made some sense )

if(!Auth::attempt(Request::only(['username','password']))){  //returns false
                return redirect($this->redirectPath);   /// redirect to login page
            }else{
                return redirect($this->redirectTo);   /// redirect to user list page
            }

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