Adding a calculated field to a Query at run time

Question

I'm getting data using a query in Delphi, and would like to add a calculated field to the query before it runs. The calculated field is using values in code as well as the query so I can't just calculate it in SQL.

I know I can attach an OnCalcFields Event to actually make the calculation, but the problem is after adding the calculated field there are no other fields in the query...

I did some digging and found that all of the field defs are created but the actual fields are only created

if DefaultFields then
    CreateFields

Default Fields is specified

procedure TDataSet.DoInternalOpen;
begin
    FDefaultFields := FieldCount = 0;
    ...
end;

Which would indicate that if you add fields you only get the fields you added.

I would like all the fields in the query AS WELL AS the ones I Add.

Is this possible or do I have to add all the fields I'm using as well?

Answer 1

If you have know your to be calculated fields names at runtime, you can use something like that.

var
 initing:boolean;

procedure TSampleForm.dsSampleAfterOpen(
  DataSet: TDataSet);
var
 i:integer;
 dmp:tfield;
begin
if not initing then
 try
  initing:=true;
  dataset.active:=false;
  dataset.FieldDefs.Update;
  for i:=0 to dataset.FieldDefs.Count-1 do
  begin
   dmp:=DataSet.FieldDefs.Items[i].FieldClass.Create(self);
   dmp.FieldName:=DataSet.FieldDefs.Items[i].DisplayName;
   dmp.DataSet:=dataset;
   if (dmp.fieldname='txtState') or (dmp.FieldName='txtOldState') then
   begin
     dmp.Calculated:=true;
     dmp.DisplayWidth:=255;
     dmp.size:=255;
   end;
  end;
  dataset.active:=true;
 finally
  initing:=false;
 end;
end;

procedure TSampleForm.dsSampleAfterClose(
  DataSet: TDataSet);
var
 i:integer;
 dmp:TField;
begin
if not initing then
begin
 for i:=DataSet.FieldCount-1 downto 0 do
 begin
  dmp:=pointer(DataSet.Fields.Fields[i]);
  DataSet.Fields.Fields[i].DataSet:=nil;
  freeandnil(dmp);
 end;
 DataSet.FieldDefs.Clear;
end;
end;

procedure TSampleForm.dsSampleCalcFields(
  DataSet: TDataSet);
var
 tmpdurum,tmpOldDurum:integer;
begin
  if not initing then
    begin
      tmpDurum := dataset.FieldByName( 'state' ).AsInteger;
      tmpOldDurum:= dataset.FieldByName( 'oldstate' ).AsInteger;
      dataset.FieldByName( 'txtState' ).AsString := State2Text(tmpDurum);
      dataset.FieldByName( 'txtOldState' ).AsString := State2Text(tmpOldDurum);
    end;
end;

procedure TSampleForm.btnOpenClick(Sender: TObject);
begin
 if dsSample.Active then
   dsSample.Close;
 dsSample.SQL.text:='select id,state,oldstate,"" as txtState,"" as txtOldState from states where active=1';
 dsSample.Open;
end;