Firebase Firestore: Append/Remove items from document array

Question

I am trying to append/remove items from an array inside of a Firestore Document but every time the entire array is replaced instead of the new value being appended. I have tried both of the following:

batch.setData(["favorites": [user.uid]], forDocument: bookRef, options: SetOptions.merge())

batch.updateData(["favorites": [user.uid]], forDocument: bookRef)

I know that instead of an array I can use an object/dictionary but that would mean storing additional data that is irrelevant (such as the key), all I need is the ID's stored inside the array. Is this something that is currently possible in Firestore?

Answer 1

Update elements in an array

If your document contains an array field, you can use arrayUnion() and arrayRemove() to add and remove elements. arrayUnion() adds elements to an array but only elements not already present. arrayRemove() removes all instances of each given element.

let washingtonRef = db.collection("cities").document("DC")

// Atomically add a new region to the "regions" array field.
washingtonRef.updateData([
    "regions": FieldValue.arrayUnion(["greater_virginia"])
])

// Atomically remove a region from the "regions" array field.
washingtonRef.updateData([
    "regions": FieldValue.arrayRemove(["east_coast"])
])

See documentation here