Delphi - TClientDataSet: Second call to applyupdates() does not apply the updates

Again I have a problem with the TClientDataSet. I guess it's something really simple but I struggle on it for a while now.

Here's some code what shows what I want to do:

procedure TForm1.Button1Click(Sender: TObject);
begin
  ClientDataSet1.Insert;
  ClientDataSet1.FieldByName('anruf_von').AsDateTime := time;
  ClientDataSet1.Post;
  ClientDataSet1.ApplyUpdates(0); // without this applyUpdates in button2 works. 
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  ClientDataSet1.edit;
  ClientDataSet1.FieldByName('anruf_bis').A开发者_StackOverflowsDateTime := time;
  ClientDataSet1.Post;
  showmessage(intToStr(ClientDataSet1.ChangeCount)); // returns 1
  if ClientDataSet1.ChangeCount > 0 then
    ClientDataSet1.applyUpdates(0);
end;

The code is self explaining I think. When I press button1, a record is created and after the call to applyUpdates its written to the databse. When I press button2, I want to make a change to this record and apply the updates to the database - and that doesn't work. But when I comment out the applyUpdates in button1, the applyUpdates in button2 works correctly.

Try to change Provider.UpdateMode to upWhereKeyOnly, and set key field in Provider.OnUpdateData.

My gues is that insert works always since it is executed as

 INSERT INTO ATABLE (anruf_von, anruf_bis) VALUES (...)

But update fails, since WHERE part will match DB stored time with time from clientdataset. In fact, you will probably try to match two doubles, which is a no-no.

 UPDATE ATABLE SET anruf_bis=<Time> 
 WHERE anruf_von=<WRONG Time, with more precision than stored in db>

When you set UpdateMode to upWhereKeyOnly, generated SQL sholud look like this

 UPDATE ATABLE SET anruf_bis=<Time> 
 WHERE ID=<ID Value>