Delphi REST.JSON TJson.JsonToObject Problem with inheritance - json

Question

I would like to use the REST.JSON Librairy to convert an Object to a JSON-String and back, but i'm encountering some problems.
Let's say the classes i would like to convert is "TFooChild1" and "TFooChild2" which both descend from "TFoo".
The classes look like this:
TFoo = class
protected
Name: string;
Value: Double;
end;
TFooChild1 = class(TFoo)
private
Limit: Double;
end;
TFooChild2 = class(TFoo)
private
Limit: Double;
WorkerID: Integer;
end;
Creating and converting to JSON would look something like this:
var
Foo: TFoo;
s: string;
begin
Foo := TFooChild1.Create;
Foo.Name:= '...';
... //assign all the Fields
s := TJson.ObjectToJsonString(Foo);
Foo.Free;
//conversion to string is correct...
Foo := TJson.JsonToObject<TFoo>(s, []);
//Field "Limit" is lost
end
I'm aware that this (TJson.JsonToObject<TFoo>(s, [])) would never return the type TFooChild1, but that's exactly what i need.
When you convert back to an Object, the Fields of the child classes are lost. How do I avoid this? I can't do JsonToObject<TFooChild1> because I don't know if it's gonna be Child1 or Child2.
Any hints?
I've googled if there's maybe a ConvertOption that would include Type information, but i haven't found anything

Answer 1

Add Rest.JsonReflect to your uses class as you will need to use the [JsonReflect(ctObject,ctObject,TFooInterceptor)] attribute attached to where you are referencing the TFoo class. Then you will have to code a TFooInterceptor object to intercept and handle the appropriate conversion. I would code the ObjectReverter and ObjectConverter overrides and start coding from there. Your TFooInterceptor should descend from TJSONInterceptor. Some examples are in the source REST.Json.Interceptors.pas.