Update Firestore document where user_id is same as currentUser uid - firebase

Question

I have the function that I am using to create or update customer.
I am able to successfully write to db. I created a field called user_id where I save the currentUser uid into so I can read only logged in user documents.
However, I am unable to update the documents because I know I'm probably not referencing the document the right way.
I get the following error:
flutter: Error: PlatformException(Error 5, FIRFirestoreErrorDomain, No
document to update:
What am I doing wrong?
Here's my function below:
Future createOrUpdateCustomer(Customer customer, bool isUpdating) async {
FirebaseUser user = await FirebaseAuth.instance.currentUser();
String userId = user.uid;
print('Current logged in user uid is: $userId');
CollectionReference customerRef =
await Firestore.instance.collection('customers');
if (isUpdating) {
customer.updatedAt = Timestamp.now();
customer.userId = userId;
await customerRef.document().updateData(customer.toMap());
print('updated customer with id: ${customer.id}');
print('updated customer with logged in uid: ${customer.userId}');
} else {
customer.createdAt = Timestamp.now();
DocumentReference documentReference = customerRef.document();
customer.id = documentReference.documentID;
customer.userId = userId;
print('created customer successfully with id: ${customer.id}');
await documentReference.setData(customer.toMap(), merge: true);
addCustomer(customer);
}
notifyListeners();
}

Answer 1

You are trying to update a nonexistent document. In this line,
await customerRef.document().updateData(customer.toMap())
You are creating a document reference with a randomly-generated id. You should explicitly set the id of the document you're updating.

Answer 2

I think you can update the document with conditions but the reference must be the doc id, that you see in 2nd section of firestore interface.