private function assert_validity($value, $boolean, $msg = null)
 {
     $book = new BookNumericality();
     $book->numeric_test = $value;
     if ($boolean == 'valid') {
         $this->assert_true($book->save());
         $this->assert_false($book->errors->is_invalid('numeric_test'));
     } else {
         $this->assert_false($book->save());
         $this->assert_true($book->errors->is_invalid('numeric_test'));
         if (!is_null($msg)) {
             $this->assert_same($msg, $book->errors->on('numeric_test'));
         }
     }
 }