Don't erase datapoints when used in triggers.

datapoint.go

@@ -283,6 +283,37 @@ func DatapointRetrieve(id int, name string) (dp Datapoint, err error) { // {{{
 	return
 } // }}}
 func DatapointDelete(id int) (err error) { // {{{
+	var dpName string
+	row := service.Db.Conn.QueryRow(`SELECT name FROM public.datapoint WHERE id = $1`, id)
+	err = row.Scan(&dpName)
+	if err != nil {
+		err = werr.Wrap(err).WithData(id)
+		return
+	}
+
+	var rows *sql.Rows
+	rows, err = service.Db.Conn.Query(`SELECT name FROM public.trigger WHERE datapoints ? $1`, dpName)
+	if err != nil {
+		err = werr.Wrap(err).WithData(dpName)
+		return
+	}
+	defer rows.Close()
+
+	var triggerNames []string
+	var name string
+	for rows.Next() {
+		err = rows.Scan(&name)
+		if err != nil {
+			err = werr.Wrap(err)
+			return
+		}
+		triggerNames = append(triggerNames, name)
+	}
+
+	if len(triggerNames) > 0 {
+		return werr.New("Datapoint '%s' used in the following triggers: %s", dpName, strings.Join(triggerNames, ", "))
+	}
+
 	_, err = service.Db.Conn.Exec(`DELETE FROM datapoint WHERE id=$1`, id)
 	if err != nil {
 		err = werr.Wrap(err).WithData(id)