Faster way to move data from a dataset structure to another (in TDatasetProvider)

Question

I have an custom TDatasetProvider that allows to create new fields to whatever data it provides.

So, let's say you got the folowing fields on the original dataset:

CustomerId
Name
Age

And you need to select it on DBGrid using showing an Bitmap. Well, you can since my DSP can add an boolean field called Selected to the dataset data.

The way I do that now:

Create 2 TClientDataset objects (Origin and Target)
In Origin, I load the data got from the parameters of the InternalGetRecords method (I overrode it)
In Target, I create the fielddefs defined from the Origin dataset and add the fielddefs created in design-time by the developer
Execute an CreateDataset on Target
And, row-by-row (and field-by-field), I copy the data from the Origin database to the Target dataset
In the end, return the Data variant as a return value from InternalGetRecords.

I really don't know if there's a more elegant (and faster) way to do that. There's another (faster and/or elegant) way to get that result?

can't you simply create internalcalc boolean field right on original dataset without all that mess? — vavan
After I made this component, life became easier as I can create data fields that the CDS recognize as data fields.. — Fabricio Araujo
but you doing lot of unnecessary work. and what's the point of creating those data fields? — vavan
They simplify a lot work with customizations of TDbGrid to get the functionality I want. I use it also to mark if the record is imported and order based on the main entity of the result set (which is denormalized)... And the list goes on... — Fabricio Araujo
I believe you could achieve the same using internalcalc fields and without copying the data inefficient way — vavan

Ondrej Kelle Ondrej Kelle · Accepted Answer · 2012-05-01T14:59:55

It seems that after loading the data from the source dataset, you can call IDSBase.AddField to add more fields:

uses
  DB, DBCommon, DBClient, DSIntf;

type
  THackClientDataSet = class(TClientDataSet);

procedure EncodeFieldDesc(var FieldDesc: DSFLDDesc;
  const Name: string; DataType: TFieldType; Size, Precision: Integer;
  Calculated: Boolean; Attributes: TFieldAttributes);
begin
  // ... copied from TClientDataSet.EncodeFieldDesc
end;

//...
var
  FldDesc: DSFLDDesc;
begin
  FillChar(FldDesc, SizeOf(FldDesc), 0);
  EncodeFieldDesc(FldDesc, 'SELECTED', ftBoolean, 0, 0, False, []);
  with THackClientDataSet(DataSet) do
    Check(DSBase.AddField(@FldDesc));
  // now you can create a second client dataset and assign it DataSet.Data directly:
  // DataSet2.Data := DataSet.Data;
  // DataSet2 now contains the new field (with empty values in all existing records)
end;

I didn't test it thoroughly but the simple example above worked as expected, I was able to navigate the second client dataset and edit the values of all fields as usual.

Faster way to move data from a dataset structure to another (in TDatasetProvider)

4 Answers