/**
  * Store a newly created resource in storage.
  *
  * @param  \Illuminate\Http\Request  $request
  * @return \Illuminate\Http\Response
  */
 public function store(Request $request)
 {
     $this->validate($request, ['book_id' => 'required|exists:books,id']);
     $assignment = BookAssignment::create(['user_id' => $request->user()->id, 'book_id' => $request->input('book_id')]);
     AssignmentProgress::create(['assignment_id' => $assignment->id, 'num_pages_read' => 0]);
     return response(['assignment' => $assignment], 201)->header('Location', '/api/assignments/' . $assignment->id);
 }