Firestore now supports "IN" queries:
Announcement
Documentation
Example:
let citiesRef = db.collection("cities")
citiesRef.whereField("country", in: ["USA", "Japan"])
Before November 2019
In Firestore, there is no "where in" like you might be used to with SQL.
If you know the values you want to query, perform different queries for each one, and call getDocument() on each of the DocumentReference objects. You typically would do this in a loop an collect the results yourself.
