Question

I am looking for a way to provide a ListSource to a TDBLookupComboBox in delphi without having an actual table on the database server to draw that list from. The DataField for the Combo Box is a 1 character field that contains a coded value such as ‘A’ = ‘Drivers License’, ‘B’ = ‘Passport’, ‘C’ = ‘Library Card’, etc. That is to say that the table only contains A, B, or C. The application is responsible for Displaying ‘Drivers License’ in the GUI. Normally a database might have a look up table but this database does not and I can not add one. My idea is that the DataSource and ListSource for a DB Look-up control do not have to be the same database, so if it were possible to define a small table in my form that contains the look-up data then I could use that an not require a real database table.

Does anyone know of a delphi component that allows a TDataSet to be defined on a form without having any actual data files behind it?

Answer 1

An alternative solution is to use TComboBox rather than TDBLookupComboBox. Use a TDictionary to define a simple in memory lookup.

type
  TMyForm = class(TForm)
    MyComboBox: TComboBox;
    MyDataset: TSimpleDataSet;
    procedure MyComboBoxChange(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    ComboLookup: TDictionary<string, Char>;
  end;

implementation

{$R *.dfm}

procedure TMyForm.FormCreate(Sender: TObject);
var
  Key: string;
begin
  ComboLookup := TDictionary<string, Char>.Create;
  ComboLookup.Add('Drivers License', 'A');
  ComboLookup.Add('Passport', 'B');
  ComboLookup.Add('Library Card', 'C');
  for Key in ComboLookup.Keys do
  begin
    MyComboBox.Items.Add(Key);
  end;
end;

procedure TMyForm.MyComboBoxChange(Sender: TObject);
begin
  // This may be wrong didn't bother to look
  //up the correct way to change a field's value in code.
  MyDataset.Fields.FieldByName('IDCard').AsString := ComboLookup[MyComboBox.Text];
end;

You could use TComboBox.Items.AddObject instead of a separate lookup table but you’d have to create a wrapper class to store a char as a TObject or use Chr to convert it to an integer then cast it to TObject but a the above is simpler in my opinion.