How can I implement “writable calculated” fields in a TDataSet?

Question

I am in the need to add additional fields to a TDataSet that don't exist in the underlying database but can be derived from existing fields. I can easily do this with caclulated fields and that works perfectly.

Now I want to edit these fields and write the changed data back. I can reverse the calculation to write the data back into the existing fields, but the DB controls just don't let me edit calculated fields.

Is there any approach that allows me to do this?

Update: Ok, some more details about the background.

The dataset has a blob field, which is a TBytes representation. Some of the bytes are identified to contain information that can be represented in a convenient way with existing DB edit fields. Not all of the bytes are known, though, so the TBytes representation has to be kept as it is for processing through another application that knows about it. This app also modifies existing and inserts new records.

The TBytes of different records in the dataset often map to different fields representations, although setting a filter or range on the dataset will ensure that they have the same mapping.

As I said, extracting the known bytes and convert it into strings, dates, numbers and so on via calculated fields is no problem. Reconverting those values into the TBytes is also possible. The problem is making those extra fields editable, while keeping the dataset navigation intact.

If it helps: We have classes that do the bidirectional mapping, exposing the fields as published properties.

Answer 1

The answer depends on a data access components you are using. I am using Anydac and it support fkInternalCalc fields, which may be as calculated as manually edited.

Answer 2

I think calculated fields are by definition read-only, with values calculated on the client. What you want could probably be implemented by an updatable view. You could define the view with calculated fields already - these would be calculated in SQL on the server - and an update trigger, maybe an insert trigger, too - to perform the reverse calculation. Then from the client you could use the view transparently like a table.

Answer 3

I had similar Issue with a ClientDataSet, i have solved this issue using dummy fileds on the SQL-Stmt so i could simulate the Fields in the Database.

See my Question

Answer 4

You can use TDatasetProvider.OnGetRecords (doesn't remember if this is the correct name of the event) and modify the datapacket sent to the Clientdataset.

Of course, you'll have to deal with them on an ApplyUpdates handler, as TOndrej said.

Answer 5

Woll2Woll's infopower components (I just tested their TwwDBEdit) allow to do such thing. So I would think whatever blocks you is at the TDBEdit level (or at the TFieldDataLink level).

What exactly differs in TwwDBEdit, I don't know. (And I'm not sure the license agreement would allow me to post here...).

Answer 6

In our database design, some values are percentages relative to another column (called oMean below), whereas other float values are stored as absolutes. Our customers later wanted both options (rel. and abs.) for all fields, so we came up with the following class derived from TFloatField. It should work for all TDataSet descendants.

unit EditableCalcFloatField;

interface

uses
  db, classes;

type
  TEditableCalcFloatField = class(TFloatField)
  public
    oAbs, oRel, oMean: TField;
  protected
    function GetCanModify: Boolean; override;
    procedure SetAsFloat(Value: Double); override;
  end;

implementation

function TEditableCalcFloatField.GetCanModify: Boolean;
begin
  Result := oMean.AsFloat <> 0;
  if not Result then Exit;
  Result := (oAbs <> nil) or (oRel <> nil);
end;

procedure TEditableCalcFloatField.SetAsFloat(Value: Double);
var
  fMean                                 : Double;
begin
  inherited;

  if DataSet.State in [dsEdit, dsInsert] then begin
    fMean := oMean.AsFloat;
    if fMean = 0 then Exit;
    if oAbs <> nil then
      oAbs.AsFloat := Value / 100 * fMean
    else
      oRel.AsFloat := Value / fMean * 100;
  end;
end;

end.

To use it without a package, you have to create the field in FormCreate, before the dataset is opened:

with TEditableCalcFloatField.Create(Self) do
begin
  oAbs := sqlMerkmaleYourAbsoluteColumn;
  DisplayLabel := sDisp;
  oMean := sqlMerkmaleAssignedValue_Mean;
  Calculated := True;
  FieldName := 'R' + oAbs.FieldName;
  DataSet := sqlMerkmale;
end;

And of course, its contents can be set either in the OnCalcFields event or by the user.

Answer 7

Use a TQuery descendant (MyQuery) with 'Select *, 0 as TempField from MyTable'

Procedure MyQueryAfterOpen(Dataset:TDataSet);
Begin
    DataSet.FieldByName('TempField').ReadOnly := False;
End;

It is now an editabe temporary field