[Flutter] MobX in Flutter/Dart: How to design a "polymorphic" reactive model?

how to design a "polymorphic" model? I have Meta with 2 children, FooMeta and BarMeta. I tried doing this:

meta.dart:

part 'meta.g.dart';

class Meta extends _Meta with _$Meta {
Meta({
required super.key,
super.label,
});
}

abstract class _Meta with Store {
String key;

@observable
String? label;

_Meta({
required this.key,
this.label,
});
}


foo_meta.dart:

part 'foo_meta.g.dart';

class FooMeta extends _FooMeta with _$FooMeta {
FooMeta({
required super.key,
super.label,
super.value,
});
}

abstract class _FooMeta extends Meta {
@observable
String? value;

_FooMeta({
required super.key,
super.label,
this.value,
});
}


But foo_meta.g.dart is never generated.

My goal is to create a hierarchy of Meta that maps to polymorphic JSON. E.g. {"key": "Foo", "value": "Some string"} mapped to FooMeta while {"key": "Bar", "value": {"rich": "data"}} mapped to BarMeta}. In Java world, this is how you'd do it: https://stackoverflow.com/a/30386694

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