Delphi - Parse TNullable (record) to JSON - json

Question

We are using the Spring4D nullable types (which are records, not objects) in some of our business objects that need to be parsed to JSON.
When the nullable type field has no value, there are 2 options that would be okay in our case:
The field is not present in the JSON
The field is present in the JSON with value null
I am trying to make this work by using a TJSONInterceptor subclass.
For example for the TNullableInteger:
I want to create an interceptor that will be used when the field has a NullableIntegerAttribute (derived from JsonReflectAttribute), in which case my TNullableIntegerInterceptor will be used.
The problem is that I don't quite know which convertertype and revertertype to use in this case because the nullable types are record types and not object types.
Does anyone have any experience with parsing record types in Delphi?
Or are there other ways to achieve this?
Any guidance would be much appreciated.
UPDATE
In the meantime I have managed to make interceptors for most of the TNullable types that we use.
Example for TNullableInteger:
{ JsonNullableIntegerAttribute }
constructor JsonNullableIntegerAttribute.Create;
begin
inherited Create(ctObject, rtString, TNullableIntegerInterceptor);
end;
{ TNullableIntegerInterceptor }
constructor TNullableIntegerInterceptor.Create;
begin
inherited;
ConverterType := ctObject;
ReverterType := rtString;
end;
function TNullableIntegerInterceptor.ObjectConverter(Data: TObject; Field: string): TObject;
var
ctx: TRTTIContext;
Int: TNullableInteger;
begin
Int := ctx.GetType(Data.ClassType).GetField(Field).GetValue(Data).AsType<TNullableInteger>;
If Int.HasValue
then begin
StringProxy.Value := IntToStr(Int.GetValueOrDefault);
Result := StringProxy;
end
else Result := nil;
end;
procedure TNullableIntegerInterceptor.StringReverter(Data: TObject; Field, Arg: string);
var
ctx: TRTTIContext;
Value: TValue;
Int: Integer;
begin
If TryStrToInt(Arg, Int)
then begin
TValue.Make<TNullableInteger>(Int, Value);
ctx.GetType(Data.ClassType).GetField(Field).SetValue(Data, Value);
end;
end;
The only type that is still a problem is TNullableBoolean.
For the unmarshalling the procedure TJONUnMarshal.PopulateFields checks if there is a reverter available but then, depending on the revertertype, it only handles TJSONArray, TJSONObject, TJSONString and TJSONNull. Boolean values in JSON are of type TJSONBool and are not handled. Instead I get the error 'JSON object expected for field fieldname in JSON true'
Also, for the serialization, I don't know how I can achieve that my JSON looks like
"FieldName": true
instead of
"FieldName": "true"
Passing a boolean as a string in JSON is not allowed by our backend software. So for now I am using a rather stupid workaround in my ObjectConverter method by returning "BOOLEAN_TRUE_IDENTIFIER" or "BOOLEAN_FALSE_INDENTIFIER" which I replace by true or false in the resulting JSON just before sending it to our backend.
If anyone would have a solution for this, it would make me very happy :-)
In the meantime we will try to avoid using NullableBoolean variables.