addDocument()

    public function addDocument($title, $date, $file, $subject_id) {

        if (!Validation::validTitle($title)) {
            throw new InvalidInputException(Messages::$invalid_title);
        }

        if (!Validation::validDate($date)) {
            throw new InvalidInputException(Messages::$invalid_date);
        }

        if ($file["name"] == null) {
            throw new InvalidInputException(Messages::$no_file);
        }

        if ($file["error"] > 0) {
            $error = $file["error"];

            switch ($error) {
                case 1:  $message = Messages::$file_too_big; break;
                default: $message = Messages::$system_error;
            }

            throw new InvalidInputException($message);

        } else {

            //$extension = end(explode(".", $file["name"]));
            $extension = strtoupper(substr(strrchr($file["name"],'.'),1));

            if (!Validation::validFileType($extension)) {
                throw new InvalidInputException(Messages::$invalid_extension);
            }
            if ($file["size"] > Config::$max_file_size*1024*1024) {
                throw new InvalidInputException(Messages::$file_too_big);
            }

                    $file["name"] = time()."_".$file["name"];
            move_uploaded_file($file["tmp_name"], "../uploads/" . $file["name"]);

            $file_name = $file["name"];
            $size = $file["size"];

            $sql = "INSERT INTO documents
                        (title, date, file_name, extension, size, subject_id)
                    VALUES
                        ('$title','$date','$file_name','$extension','$size','$subject_id')";

             $result = $this->query($sql);

             if (!$result) {
                throw new DatabaseErrorException(mysql_error());
             }

        }

        return true;

    }