How to find out which field was changed in a delphi 6 TZTable via ZeosLib

Question

I have a TZTable (ZEOSlib) bound to a DBGrid now I need to know which particular TField was changed by the user.

I tried with

if NOT (taPositionenArtNrGH.NewValue = taPositionenArtNrGH.OldValue) then
    ShowMessage('ArticleNumber changed');

I placed the code in

• BeforePost, OnUpdateRecord, AfterPost

But in the Debugger OldValue is always NewValue. How do I check which field was changed?

Answer 1

You can use UpdateStatus : TUpdateStatus for this. For example:

1. Set ZTable.CachedUpdates to true;
2. Create new calculated field named "Status".
3. To show old value for example of field "FNAME" create new calculate field named "FNameOldValue"

4. In OnCalcFields event use:

   procedure TDM1.ZTable1CalcFields(DataSet: TDataSet);
   begin
     if ZTable1.UpdateStatus in [usModified] then
       begin
         ZTable1Status.value := 'Modified';
         ZTable1FNameOldValue.value := ZTable1FNAME.OldValue;
       end
     else
       ZTable1Status.value := 'UnModified'
   end;
   

Result :

Edit:

You can detect field level changes like:

if ZTable1.UpdateStatus in [usModified] then
  begin
    for I := 0 to ZTable1.Fields.Count - 1 do
      begin
        if ZTable1.Fields[i].OldValue <> ZTable1.Fields[i].NewValue  then
          -- do something with this field
      end;
   end; 

Answer 2

As per documentation:

The NewValue property is only usable when the data is accessed using a TClientDataSet component or cached updates is enabled.

Answer 3

If you just want to know what fields have been changed, why not use TField.OnChange event? You could fill a list of field names in this event and clear it in OnAfterPost. But the Modified property would be very useful indeed; it's odd that it haven't been implemented yet.