FFieldMapper is null

WEB TMS WEB Core
1

I am getting an exception that was making no sense.

I traced it. Its ion this code, unit JSONDataset

function TBaseJSONDataSet.GetFieldData(Field: TField; Buffer: TDatarecord): JSValue;

var
  R : JSValue;

begin
  if State in [dsCalcFields,dsInternalCalc] then
    R:=CalcBuffer.data
  else if (State=dsFilter) then
    R:=FFilterRow
  else if (FEditIdx=Buffer.Bookmark) then
    begin
    if State=dsOldValue then
      R:=Buffer.data
    else
      R:=FEditRow;
    end
  else
    begin
    if State=dsOldValue then
      Exit(Null)
    else
      R:=Buffer.data;
    end;
  Result:=FFieldMapper.GetJSONDataForField(Field,R);   <-------------------
  if isUndefined(Result) then
    Result:=Null;
end;

FFieldmapper is null. What does this mean? What havent I done ?

The origin code is

  edtRegistration.Text := xds.FieldByName (fld_AC_Rego).AsString;

And the debugger shows that TXdataWebDataset has the data and that Rego is not null.

17.09.2025_16.48.59_REC
17.09.2025_16.48.59_REC290×525 7.05 KB

2

FFieldMapper is created in the dataset.Open method.
Are you trying to use this without having called Open first?

3

I am using Load after setting a filter. I was setting up 5 of these datasets and then a small timer delay. This worked in Insert mode of the main table. But was failing in View or Edit mode.

On a hunch, I extended my timer and it works.

Is there anything else I can use apart from LoadAsync ? I assume that this halts the "thread" until the dataset is loaded, which is not ideal.

4

The proper way is to use await with LoadAsync if you do operations after the loading.

5

Does this freeze the execution of the code though?

On reading the docs, it would seem that if I use Load, it returns right away. But calls Open when its loaded the data. So, could I use AfterOpen event ???

6

You could indeed use the AfterOpen event to ensure that this code is executed only after the open effectively finished.

7

Thanks, I just tested it, it seems to work.