Firebase nested HttpsCallableResult to dart model - json

Question

I'm trying to get some data from a cloud function and assign it to a model. But I am unable to used the nested data, I get the following error:
_TypeError (type '_InternalLinkedHashMap<Object?, Object?>' is not a subtype of type 'Map<String, dynamic>')
The data I receive looks like this:
{
"answer": "Some optional answer",
"error": "Some optional error",
"usage": { "prompt_tokens": 32, "completion_tokens": 40, "total_tokens": 72 }
}
When I receive the data I try to assign it to a model:
final HttpsCallableResult result = await functions
.httpsCallable('askHW')
.call({'question': userQuestion});
return HWResponse.fromJson(result.data);
HWResoponse:
class HWResponse {
final String answer;
final String error;
final HWUsage usage;
HWResponse({this.answer = '', this.error = '', required this.usage});
factory HWResponse.fromJson(Map<String, dynamic> json) => HWResponse(
answer: json.containsKey('answer') ? json['answer'] as String : '',
error: json.containsKey('error') ? json['error'] as String : '',
usage:
json["usage"] == null ? HWUsage() : HWUsage.fromJson(json["usage"]),
);
Map<String, dynamic> toJson() => {
"answer": answer,
"error": error,
"usage": usage.toJson(),
};
}
class HWUsage {
final int promptTokens;
final int completionTokens;
final int totalTokens;
HWUsage({
this.promptTokens = 0,
this.completionTokens = 0,
this.totalTokens = 0,
});
factory HWUsage.fromJson(Map<String, dynamic> json) => HWUsage(
promptTokens: json.containsKey('prompt_tokens')
? json['prompt_tokens'] as int
: 0,
completionTokens: json.containsKey('completion_tokens')
? json['completion_tokens'] as int
: 0,
totalTokens:
json.containsKey('total_tokens') ? json['total_tokens'] as int : 0,
);
Map<String, dynamic> toJson() => {
"prompt_tokens": promptTokens,
"completion_tokens": completionTokens,
"total_tokens": totalTokens,
};
}

Answer 1

Using Map<String, dynamic>.from('json['usage]') in HWResponse on the nested field seems to be the correct way to do this. To avoid any errors I also used json.containsKey('usage') to make sure that the nested field usage actually exists.
usage: json.containsKey('usage')
? HWUsage.fromJson(Map<String, dynamic>.from(json['usage']))
: HWUsage(),