I'm getting data using a query in Delphi, and would like to add a calculated field to the query before it runs. The calculated field is using values in code as well as the query so I can't just calculate it in SQL.
I know I can attach an OnCalcFields Event to actually make the calculation, but the problem is after adding the calculated field there are no other fields in the query...
I did some digging and found that all of the field defs are created but the actual fields are only created
if DefaultFields then
CreateFields
Default Fields is specified
procedure TDataSet.DoInternalOpen;
begin
FDefaultFields := FieldCount = 0;
...
end;
Which would indicate that if you add fields you only get the fields you added.
I would like all the fields in the query AS WELL AS the ones I Add.
Is this possible or do I have to add all the fields I'm using as well?
Delphi now has the option to combine automatic generated fields and calculated fields : Data.DB.TFieldOptions.AutoCreateMode an enumeration of type TFieldsAutoCreationMode. This way you can add your calculated fields at runtime. Francois wrote in his answer how to add a field at runtime.
Different modes of TFieldsAutoCreationMode :
acExclusive
When there are no persistent fields at all, then automatic fields are created. This is the default mode.
acCombineComputed
The automatic fields are created when the dataset has no persistent fields or there are only calculated persistent fields. This is a convenient way to create the persistent calculated fields at design time and let the dataset create automatic data fields.
acCombineAlways
Automatic fields for the database fields will be created when there are no persistent fields.