delphi - How can I use a DBLookupComboBox to select a foreign key value without immediately changing the target table? (Delphi 5 Pro)

Question

Basic Scenario:

1. User clicks 'edit item'
2. Edit dialog opens
3. A combobox 'item type' should be populated with items form a table 'item_type'. The combobox should display 'item_type.name', but also know about 'item_type.id'
4. User edits other item stuff and selects an item type, then clicks ok
5. My program does some input validation
6. if everything is ok, get the 'item_type.id' from the selected combo item and save it to the foreign key column of the item table ('item.fk_item_type').

If I understand this component correctly, I should set DataSource to point to the destination table 'item' and DataField to 'item.fk_item_type'. But that would immediately edit my item table before I get a chance the validate all the other input.

It feels like I am missing something here. Somewhere I read that I need to use a classic ComboBox and fill it manually. But then I don't understand how to get to the id of the selected item.

Thank you for any pointers.

Edit: I am starting to suspect that maybe I am missing a fundamental thing. All these DB* components, do they load values from that database automatically, but I have to call Post() myself? Meaning they do not automatically change values in the database?

Answer 1

If I understand you correctly, you want to use a DBLookupComboBox. You have to supply values for the following properties

• datasource - linked to the table which you are editing, presumably 'items'
• datafield - the name of the field in the table which you are editing, presumably 'item_type'
• listsource - linked to the table which populated the combobox, presumably 'item_types'
• list field - the name of the field from 'item_types' which you want to display, presumably 'name'
• key field - the name of the field from 'item_types' which will be inserted into the items record, presumably 'item_type'

The table which populated the combobox is never edited.

You can validate the values before posting the new/edited 'items' record.

I can show you how to use a non-data aware combobox if necessary, but it's easier to use the data aware version.

Regarding validation, I use the following code template in edit dialogs.

Function TEditQuestion.Execute (n: longint): boolean;
var
 gen: longint;

begin
 sdsEditQuestion.params[0].asinteger:= n;  // TSQLDataSet
 with qEditQuestion do                     // TClientDataSet
  begin
   open;
   if n = -1 then                          // new record
    begin
     caption:= 'New record';
     insert;
     fieldbyname ('alive').asinteger:= 1;
     // initialise necessary fields in the dataset
    end
   else caption:= 'Editing record ' + inttostr (n);

   edit;
   if showmodal = mrOK then
    begin
     // validation code comes here. Set 'result' to true if everything is ok
     if result then
      begin
       if n = -1 then
        begin
         with qGenID do
          begin
           open;
           gen:= fields[0].asinteger;    // get new value from generator
           close
          end;

         FieldByName ('id').asinteger:= gen;
        end;
       post;
       applyupdates (0)
      end
     else cancel  // showmodal = OK, result = false
   else           // showmodal cancelled
    begin
     cancel;
     result:= false
    end;
  end             // with qEditQuestion
 end;