[firestore_database]: Cannot store DocumentReference as map field key

[paulchirita]

Is there an existing issue for this?

• I have searched the existing issues.

Which plugins are affected?

Database

Which platforms are affected?

iOS, macOS, Web, Windows, Android

Description

Actual behavior

When using a DocumentReference as a map field key and attempting to write that map field to a Firestore document, the SDK throws a type cast error:
type '_JsonDocumentReference' is not a subtype of type 'String' in type cast

Expected behavior

Since Firestore natively supports DocumentReference as map keys (this can be verified by manually adding a reference-type map key via the Firebase Console UI), the FlutterFire SDK should serialize DocumentReference keys the same way it handles DocumentReference values.

Reproducing the issue

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:flutter/material.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  final firestore = FirebaseFirestore.instance;
  final docRef = firestore.doc('testCollection/testDoc');
  final Map<DocumentReference, double> doublePerRef = {
    docRef: 100.0,
  };
  try {
    await firestore.doc('testCollection/testDoc').set({
      'doublePerRef': doublePerRef,
    });
    print('Success');
  } catch (e) {
    print('Error: $e');
    // Error: type '_JsonDocumentReference' is not a subtype of type 'String' in type cast
  }
}

Firebase Core version

3.15.2

Flutter Version

3.35.7

Relevant Log Output

Flutter dependencies

No response

Additional context and comments

No response