how to set custom object with document id to a collection in firestore in flutter?

I defined a dart class A with some string variables and a list of objects of another class B. I don't find any method function in cloud_firestore module to set this custom object of class A, as a document to the collection, even though custom objects are supported in Firestore. Do I need to convert all the members(strings, lists etc) of class A to JSON format or any other solution available?

In the event that it's inconvenient to create another collection of the related custom class, the only alternative is to build the related class instances yourself.

import Player from // player module

class GameState {
  constructor(data) {
    this.players = data.players.map(p => new Player(p))
    // ...
  }

  // flatten GameState into a generic JS map
  // build one of these on Player also
  asFBData() {
    const playerFBData = this.players.map(p => p.asFBData())
    return { playerFBData, ...other_game_state_here }
  }
}

const gameStateConverter = {
  toFirestore: gameState => gameState.asFBData(),
  fromFirestore: (snapshot, options) => new GameState(snapshot.data(options))
}