Use Transaction in MIDAS

This is a article I found from Mr. Chen Jiangyong on Use of Transaction with MIDAS:

 

Transaction is very popular used in database programming. It's very easier used in Two-Tier application. How do it in Three-Tier? I have try to use cache-update in remote data modules and use transaction, I test it successfully in Delphi 4. But to do it cost many code and low performance. When Delphi 5 coming, I try to use the same method, but failed. Maybe this is the bug of Delphi 5. I think that many person have the same situation with me. How can use transaction in MIDAS? I try another method, just do it at BeforeUpdateRecord event. For example, when I edit table A, I want to insert a log to another table B. To do this, first:

1) create a remote data module , drop a query to select records from table A, drop a TUpdateSQL for the query, specify the SQL statement of Table A. Drop a TDatasetprovider hook to the query. Drop another query, then in its SQL property write the insert SQL statement to table B.

2) In the BeforeUpdateRecord event, I write this code:

  SetParams(UpdateSQL1, DeltaDS, UpdateKind);   
  UpdateSQL1. ExecSQL(UpdateKind);
  if UpdateKind = ukUpdate then begin
    { Insert a log to Table B}
    // set the params ­
    Query2.ExecSQL;
  end;
  Applied := True;

3) In the client, you should also make some change. When you try to save the changes, use ApplyUpdates(0) instead of ApplyUpdates(-1). For example:

  ClientDataSet1.ApplyUpdates(0)

  This is OK. Delphi does it well. If a error occur when you try to insert a log to Table B, the transaction will rollback, then you can find that Table A's record is not changed at all.
  I have tested it at Delphi 4, It also works very well.

Good Luck.