use pointers for optional update fields to distinguish empty from unset

cmd/update.go

@@ -61,8 +61,17 @@ var updateCmd = &cobra.Command{
 			uRmTags = append(uRmTags, "_prio::low", "_prio::medium", "_prio::high")
 		}
 
-		uParams := db.UpdateParams{Title: uTitle, Content: uContent, DueDate: uDue, ClearDue: uClearDue,
+		uParams := db.UpdateParams{ClearDue: uClearDue,
 			AddTags: uAddTags, RemoveTags: uRmTags, AddRels: addRels, RemoveRels: rmRels}
+		if cmd.Flags().Changed("title") {
+			uParams.Title = &uTitle
+		}
+		if cmd.Flags().Changed("content") {
+			uParams.Content = &uContent
+		}
+		if cmd.Flags().Changed("due") {
+			uParams.DueDate = &uDue
+		}
 		if err := d.UpdateNode(args[0], uParams); err != nil {
 			fmt.Fprintln(os.Stderr, "failed to update:", err)
 			return