how to write nested array in Firestore using flutter - flutter

Question

Building a shopping app I added a cart item in my 'users' collection.
cart stored in
and after the user purchase those item I wanted to store those cart items in other collection 'orders'. but when I tried to store those cart items in my order collection firebase throws Invalid data. Nested arrays are not supported.
what is my mistake here?
Here is my Code
onPressed: (){
//orderItem(cartItem);
getData() async {
return await FirebaseFirestore.instance.collection('users').where('id', isEqualTo: FirebaseAuth.instance.currentUser.uid).get();
}
getData().then((val){
if(val.docs.length>0){
//print(val.docs[0].data()['cart']);
// Map map = val.docs[0].data()['cart'];
var list = [val.docs[0].data()['cart']];
// map.entries.forEach((e) {
// list.add();
// });
// List items = [val.docs[0].data()['cart'].toList()];
FirebaseFirestore.instance.collection('orders').doc().set({
"userId": FirebaseAuth.instance.currentUser.uid,
"userName": FirebaseAuth.instance.currentUser.displayName,
"orders": FieldValue.arrayUnion([[val.docs[0].data()['cart']]]),
"price": cartController.totalCartPrice.value
});
}
else{
print("Not Found");
}
});
},

Answer 1

I think the problem is trivial. You accidentally put list inside list.
"orders": FieldValue.arrayUnion([[val.docs[0].data()['cart']]]),
I think you can skip one pair of [ so you have this:
"orders": FieldValue.arrayUnion([val.docs[0].data()['cart']]),
Also... if val.docs[0].data()['cart'] is a list you'll probably need to skip another pair to this:
"orders": FieldValue.arrayUnion(val.docs[0].data()['cart']),

Answer 2

Arrays in a Firestore documents cannot contain arrays so you cannot have have something like:
[
[orderId: 1, orders: [...orders]],
[orderId: 2, orders: [...orders]],
//This is possible
{orderId: 3, orders: [...orders]}
]
If you try adding a document from the console itself, you'll notice the same.
The best you can do is store the orders as a string, [JSON.stringify(ordersObject)]. Then when you fetch the data back you can parse it into as JSON again..
Second option is restructuring your data, it would have ideal to have a separate collection for orders. Then you can use .where() queries to fetch orders of a specific user. However this'll be more expensive (but ideal) because you will be fetching 12 documents (12 reads) if user has 12 orders.