// ShowError sends an appropriate error message.
func ShowError(w http.ResponseWriter, r *http.Request, err error) {
	switch e := err.(type) {
	case *pq.Error:
		switch e.Code {
		case "23502":
			// not-null constraint violation
			http.Error(w, fmt.Sprint("Some required data was left out:\n\n", e.Message), http.StatusForbidden)
			return
		case "23503":
			// foreign key violation
			switch r.Method {
			case "DELETE":
				http.Error(w, fmt.Sprint("This record can’t be deleted because another record refers to it:\n\n", e.Detail), http.StatusForbidden)
				return
			}
		case "23505":
			// unique constraint violation
			http.Error(w, fmt.Sprint("This record contains duplicated data that conflicts with what is already in the database:\n\n", e.Detail), http.StatusForbidden)
			return
		case "23514":
			// check constraint violation
			http.Error(w, fmt.Sprint("This record contains inconsistent or out-of-range data:\n\n", e.Message), http.StatusForbidden)
			return
		default:
			msg := e.Message
			if d := e.Detail; d != "" {
				msg += "\n\n" + d
			}
			if h := e.Hint; h != "" {
				msg += "\n\n" + h
			}
			http.Error(w, msg, http.StatusInternalServerError)
			return
		}
	case *strconv.NumError:
		http.Error(w, fmt.Sprintf(`"%s" is not a valid number.`, e.Num), http.StatusBadRequest)
		return
	default:
		switch err {
		case sql.ErrNoRows:
			http.NotFound(w, r)
			return
		}
	}
	http.Error(w, err.Error(), http.StatusInternalServerError)
}