What is the process for adding information to a pre-existing Firestore database using Flutter?

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1. Initialize the Firestore instance: You first need to initialize the Firestore instance within your app. This can be done in the main.dart file through the following code:

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

1. Get a reference to the document: To add information to a pre-existing document, you need to first get a reference to that document. This can be done using the Firebase API through the following code:

final FirebaseFirestore _db = FirebaseFirestore.instance;
final DocumentReference documentReference = _db.collection('collectionName').doc('documentName');

1. Create a Map of the data you want to add: You then need to create a Map of the data you want to add to the document. This can be done through the following code:

Map<String, dynamic> data = <String, dynamic>{
  'field1': 'value1',
  'field2': 'value2',
  ...
};

1. Add the data to Firestore: Finally, you can add the data to Firestore through the following code:

await documentReference.update(data);

This will add the data to the pre-existing document.