Undefined property: IlluminateDatabaseEloquentBuilder::$author

class Comment extends Model
{

    public function author()
    {
        return $this->belongsTo('AppUser','user_id');
    }
}

public function index(Request $request)
{
     $comments = Comment::query()->orderby('created_at', 'DESC');

    $id=$request->input('id');
    $name=$request->input('username');


    if(!empty($id)){
        $comments->where('id', $request->input('id') )->get();
    }
    if(!empty($name)){
        $comments->where($comments->author->username, 'LIKE', $request->input('username') )->get();
    }
    $comment = $comments->paginate(10);


    return view('comments.index')->withComment($comment);
}

<div class="panel-body">
                {!! Form::open(['route' => 'comments.index', 'method' => 'GET']) !!}
                <div class="col-md-5">
                    {!! Form::label('id', 'Search By ID:') !!}
                    {!! Form::text('id', null, array('class' => 'form-control')) !!}
                </div>
                <div class="col-md-5">
                    {!! Form::label('username', 'Search By Username:') !!}
                    {!! Form::text('username', null, array('class' => 'form-control')) !!}
                </div>
                <div class="col-md-2">
                    {!! Form::submit('Find Comments', array('class' => 'btn btn-send ')) !!}
                </div>
                {!!Form::close()!!}

            </div>
@foreach($comment as $comments)
//data
@endforeach

$comments = Comment::with(['author'  => function ($query) use($request) {
    $query->where(username, 'LIKE', $request->input('username') )
}])->orderby('created_at', 'DESC');