Can I have duplicate document IDs? - firebase

Question

My application uses dates as document Id.
Each day stores the items the user bought and their total price.
The user might buy items from his local store twice or more in a day.
How can I allow that in my flutter code?
All the questions I found, ask about how not to duplicate document ID. I guess I'm the only one who want to do this.
The code just in case:
/// Save to collection [user], document [day], the list of items in [cartMap], and the total amount [total]
static Future<void> saveToFirestore(String day, Map<GroceryItem, int> cartMap, double total) async {
bool paidStatus = false;
List<String> x = day.split('-');
assert(matches(x[2], '[0-9]{2}')); // make sure the day is two digits
assert(matches(x[1], '\\b\\w{1,9}\\b')); // make sure the month is a word between 1 and 9 length
assert(matches(x[0], '[0-9]{4}')); // make sure it's a number year of four digits
final groceries = Groceries.cartMap(cartMap).groceries;
final user = await CurrentUser.getCurrentUser();
CollectionReference ref = Firestore.instance.collection(user.email);
DocumentReference doc = ref.document(day);
final dateSnapshot = await doc.get();
Firestore.instance.runTransaction((transaction) async {
updatePaidUnpaidTotal(day, user.email, total);
if(dateSnapshot.exists) {
print('Updating old document');
List<dynamic> existingItems = dateSnapshot.data['items'];
var totalItems = existingItems + groceries;
return await transaction.update(doc, {'total': FieldValue.increment(total), 'items': totalItems, 'paidStatus': false},);
}
else {
print('New document');
return await transaction.set(doc, {'total': total, 'items': groceries, 'paidStatus': paidStatus});
}
});
}

Answer 1

I strongly recommend not using dates (or any actual data at all) in a document ID, for the reason that you're running into.
There is rarely a requirement in an app to use a specific format for a document ID. It might be convenient in some cases, but ultimately, it constrains the future expansion of your collection.
One flexible way of adding any document at all, is to simply use add() to accept a random ID for the new document. Then you can put the date in a field in the document, and use that in your queries as a filter on that field. This is going to give you much more freedom to change things up later if you want, so you are not bound the dates for IDs. There are no real performance penalties for this.
The only reason to use a non-random string as a document ID is if you need to absolutely enforce uniqueness on that value. Since your application no longer wants to enforce this, it's better to simply remove that constraint and use random IDs.

Answer 2

It is not possible to have several Firestore documents in the same (sub)collection with the same document ID.
A classical approach in your case is to create one document per day and create a subcollection which contains a document for each item the user bought during this day.
Something like:
orders (collection)
- 20200725 (doc)
- orderItems (subcollection)
- KJY651KNB9 (doc with auto generate id): {'items': totalItems, 'paidStatus': false ... }
- 20200726 (doc)
- orderItems (subcollection)
- AZEY543367 (doc with auto generate id): {'items': totalItems, 'paidStatus': false ... }
- AZEY5JKKLJ (doc with auto generate id): {'items': totalItems, 'paidStatus': false ... }
- AZEY598T36 (doc with auto generate id): {'items': totalItems, 'paidStatus': false ... }