firestore add to field array
DocumentReference washingtonRef = db.collection("cities").document("DC");
// Atomically add a new region to the "regions" array field.
washingtonRef.update("regions", FieldValue.arrayUnion("greater_virginia"));
// Atomically remove a region from the "regions" array field.
washingtonRef.update("regions", FieldValue.arrayRemove("east_coast"));