How to return a nested json response in Flask-Restx

I am trying to make an API with Flask-RestX that can show a response like this,

{
  "id": 1,
  "msg": "How are things",
  "score": 345,
  "additional": {
    "goal": 56,
    "title": "ASking how"
  }
}

when data is like this (In my case, I do not control the data format),

data = {
    "id":1,
    "msg":"How are things",
    "goal": 56,
    "score":345,
    "title":"Asking how"
}

But the response I get with the current code is wrong, it shows null values

{
  "id": 1,
  "msg": "How are things",
  "score": 345,
  "additional": {
    "goal": null,
    "title": null
  }
}

Full code --

from flask import Flask
from flask_restx import Resource, Api, fields

app = Flask(__name__)
api = Api(app)

data = {
    "id":1,
    "msg":"How are things",
    "goal": 56,
    "score":345,
    "title":"Asking how"
}

extra = api.model('Extra', {
    'goal': fields.Integer,
    'title': fields.String
    })

model = api.model('Model', {
    'id' : fields.Integer,
    'msg' : fields.String,
    'score': fields.Integer,
    'additional' : fields.Nested(extra)
  })


@api.route('/hello')
class HelloWorld(Resource):
    @api.marshal_with(model)
    def get(self):
        return data

if __name__ == '__main__':
    app.run(debug=True, port=4000)

I am completely new to Flask-RestX / Flask-RestPlus. Please tell me, how can I achieve this without changing the data format itself.

Solution

In accordance with waynetech's comment above, the following worked for me:

from flask import Flask
from flask_restx import Resource, Api, fields

app = Flask(__name__)
api = Api(app)

data = {
    "id":1,
    "msg":"How are things",
    "score":345,
    "additional": {
        "goal": 56,
        "title": "Asking how",
    }
}

extra = api.model('Extra', {
    'goal': fields.Integer,
    'title': fields.String
    })

model = api.model('Model', {
    'id' : fields.Integer,
    'msg' : fields.String,
    'score': fields.Integer,
    'additional' : fields.Nested(extra)
  })


@api.route('/hello')
class HelloWorld(Resource):

    @api.marshal_with(model)
    def get(self):
        return data

    @api.marshal_with(model)
    def post(self):
        data = api.payload

        return data

if __name__ == '__main__':
    app.run(debug=True, port=4000)

here's the quick test script I used:

from pprint import pprint
import requests

payload = {
    "id": 1,
    "msg":"Test",
    "score":345,
    "additional": {
        "goal": 56,
        "title": "Test",
    }
}

url = 'http://127.0.0.1:4000/hello'
r = requests.post(url, json=payload)

if r.status_code == 200:
    print(r.status_code)
    pprint(r.json())
else:
    print("FAIL: ", r.status_code)