Sort order in Firestore arrays

Question

I'm trying to understand arrays in Firebase a bit more. Currently, I'm storing maps in arrays, where one of the fields inside the map is a position that I can use in my mobile app to sort the array with on retrieval and show results in the order of position.

The docs on Firebase say:

Arrays are sorted by elements. If elements are equal, the arrays are sorted by length.

For example, [1, 2, 3] < [1, 2, 3, 1] < [2].

And then there's a section describing how maps are sorted as well:

Key ordering is always sorted. For example, if you write {c: "foo", a: "bar", b: "qux"} the map is sorted by key and saved as {a: "foo", b: "bar", c: "qux"}.

Map fields are sorted by key and compared by key-value pairs, first comparing the keys and then the values. If the first key-value pairs are equal, the next key-value pairs are compared, and so on. If two maps start with the same key-value pairs, then map length is considered. For example, the following maps are in ascending order:

{a: "aaa", b: "baz"}
{a: "foo", b: "bar"}
{a: "foo", b: "bar", c: "qux"}
{a: "foo", b: "baz"}
{b: "aaa", c: "baz"}
{c: "aaa"}

But then I tried this in Firestore: I jumbled up the order of the maps in the above example, and stored them in an array:

data= [{"c": "aaa"}, {"a": "aaa", "b": "baz"}, {"a": "foo", "b": "baz"}, {"b": "aaa", "c": "baz"}, {"a": "foo", "b": "bar", "c": "qux"}, {"a": "foo", "b": "bar"}]

And upon inserting into a Firestore document, the array did not get sorted! While the keys themselves do get sorted within a single Map, the elements in the array stay in the same order.

So does sorting in arrays even work when elements are Maps? Here's an example of what I'm storing in Firestore:

{
    "car_collection": {
        "models": {
            data: [
                {
                    "model": "Honda",
                    "color": "black",
                    "position": 0
                },
                {
                    "model": "Hyundai",
                    "color": "red",
                    "position": 1
                },
                {
                    "model": "Chevrolet",
                    "color": "yellow"
                    "position": 2
                }
            ]
        }
    }
}

I'm storing an additional field called "position", and the order of maps stays the same on every retrieval. Wondering if I even need to store this field, or data will be sorted in the order that I store it in.

Answer 1

Submitted a ticket to Google to improve the documentation for Array type, and I think it's helpful and accurate as seen through some smoke testing.

https://firebase.google.com/docs/firestore/manage-data/data-types

Copy-pasting the current version here:

An array cannot contain another array value as one of its elements.

Within an array, elements maintain the position assigned to them. When sorting two or more arrays, arrays are ordered based on their element values.

When comparing two arrays, the first elements of each array are compared. If the first elements are equal, then the second elements are compared and so on until a difference is found. If an array runs out of elements to compare but is equal up to that point, then the shorter array is ordered before the longer array.

For example, [1, 2, 3] < [1, 2, 3, 1] < [2]. The array [2] has the greatest first element value. The array [1, 2, 3] has elements equal to the first three elements of [1, 2, 3, 1] but is shorter in length.

So it seems you can safely expect the order of elements to be maintained in Firestore, while understanding the effects of addition/removal as well.