unable to store values in intermediate table laravel

I have a Idea and community model. A Many to many relationship exists between them. Relationships are being formed perfectly. however, i am facing one issue. An idea can be found in may communities and a community can have many ideas. So, based on this there is an intermediate table. i am unable to store community_id and idea_id in pivot table.

public function storePost(IdeaRequest $request)
{
  if ($request->hasFile('idea_image')) {
      $filename = $request->file('idea_image')->getClientOriginalName();
      $moveImage = $request->file('idea_image')->move('images/', $filename);
  }
      $idea = new Idea();
      $idea->idea_title = $request->input('idea_title');
      $idea->user_id = Auth::id();
      $idea->idea_image = $moveImage;
      $idea->idea_info = $request->input('idea_info');
      $idea->communities()->attach($request->input('selection'));
      $idea->idea_location = $request->input('idea_location');
      $idea->idea_goal = $request->input('idea_goal');
      $idea->idea_description = strip_tags($request->input('idea_description'));
      $idea->save();
      Session::set('idea_id',$idea->id);
      session()->flash('flash_message', 'Your idea has been submitted for Review, Kindly Fill this Work BreakDown of You Work');
      return redirect()->route('create_wbs');
}

This line is making issue

$idea->communities()->attach($request->input('selection'));

via Chebli Mohamed