How add calculated fields to a table at runtime?
lazarus / 16-Databases

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gato2707 · 2013-05-27 05:11

Hi everybody:

Are there a way to add calculated fields to a table at runtime?

It's simple when the FieldDefs was created at design time, but in this
way the table becomes into an static structure inside the Object
Pascal code.

I would like to create the query (Table) at runtime because the
phisycal structure of a related table may change in time. v.g.

The table used for today may have a simple structructe like:

id INTEGER,
Name VARCHAR(20),
CodeID CHAR(10)

But maybe nex time the program need access a different phisycal table
like:

id INTEGER,
Name VARCHAR(20),
SPEC INTEGER,
CodeID CHAR(10)

Or any other number of fields or kind of them

I try with the FieldDefs.CreateField samples found at Internet, but
all of them are related to Delphi who has a very different definition
for that method.

In Lazarus/fpc (http://lazarus-
ccr.sourceforge.net/docs/fcl/db/tfielddef.createfield.html) its method
only has a single argument

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gato2707 · 2013-05-29 15:39

Hi everybody:

I have been solved the way to create Calculated Fields in runtime.

I hope this solution be useful for any who has same need in the
future.

1) Create a proyect and add every unit you'll need to work wit a
database.
2) Add these two variables as global to the unit. I suppose you can
declare them as part of the unit class.

Code: [Select]var
 Form1: TForm1;

 MyFields: array of TField;
 Temf: TStringField;

implementation

{$R *.lfm}

{ TForm1 }

3) In the OnCreate event do next:

Code: [Select]procedure TForm1.Create(Sender: TObject);
var
 i:integer;
begin
//Create your db componentes as you needd
//You need open an then close the table for load the fields
 MyQ.Open;
 MyQ.Close;
 SetLength(MyFields, MyQ.FieldDefs.Count);
//Add one by one the fields to make them accesible.
 for i:=0 to MyQ.FieldDefs.Count - 1 do
   MyFields[i] := MyQ.FieldDefs[i].CreateField(MyQ);
//Create all Calculated Fields as you need
 Temf := TStringField.Create(nil);
 Temf.FieldKind := fkCalculated;
 Temf.FieldName := 'NewCalcField';
 Temf.DataSet := MyQ;
 MyQ.Open;
end;

4) OnDestroy event be sure to free all fields you create before

Code: [Select]procedure TForm1.FormDestroy(Sender: TObject);
var
 i: Integer;
begin
 for I := low(MyFields) to High(MyFields) do
   MyFields[i].Free;
 SetLength(MyFields, 0);
 Temf.Free;
 MyQ.Close;
end;