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.

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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>