public function store(Request $request)
 {
     $booksToRenew = Input::get('checkbox');
     $tmp_timestamp = strtotime('today');
     foreach ($booksToRenew as $id) {
         $record = DB::table('lends')->select('id', 'due_date', 'continued')->where('book_id', $id)->get();
         $record_id = $record[0]->id;
         $due_date = $record[0]->due_date;
         $has_renewed = $record[0]->continued;
         // 是在到期3天内续借
         $cond1 = $due_date - $tmp_timestamp <= 3 * 24 * 60 * 60;
         // 3天
         // 没有续借过
         $cond2 = !$has_renewed;
         // 没有被人预约
         $reserve = DB::table('reserves')->select('id')->where('book_id', $id)->get();
         $cond3 = empty($reserve);
         //            var_dump($cond1);
         //            var_dump($cond2);
         //            var_dump($cond3);
         if ($cond1 and $cond2 and $cond3) {
             $lend = Lend::find($record_id);
             $lend->lend_date = $tmp_timestamp;
             $lend->due_date = strtotime('+30 days', $tmp_timestamp);
             $lend->continued = 1;
             $lend->save();
         }
     }
     return Redirect::to('/user/home')->with('message_success', '续借完成!');
 }