FireBase Firestore retrieve multiple documents by list of ID's

Question

I've got two collections for example Cities and Houses. Houses are not a nested collection of Cities, but a house contains a reference to a City.

Wat is the most efficient way to get city information for each house in a list?

For Example

class City {
  var name: String = ""
  var property1: String = ""
  var property2: String = ""
}

class House {
  var cityId: String = ""
  var propertyA: String = ""
  var propertyB: String = ""
}

I've got a list of houses, how do I get for each house in this list the name of the city? Is this possible by query, or should I just get all the cities (or each one seperate) and map them in code.

The cities and houses are just an example, but simular to my real situation.

Answer 1

You can use com.google.firebase.firestore.FieldPath. This is working for me in kotlin:

//friendsIDs is ArrayList<String>
    firestore!!.collection("users").whereIn(FieldPath.documentId(), friendsIDs).get().addOnCompleteListener {usersDocs ->
        if (usersDocs.isSuccessful) {
            val result = usersDocs.result!!
            val docs = result.documents
            for (userDoc in docs) {
                val friendData = userDoc.data!!
                //...
            }
        } else {
            Log.e("FirestoreRequest", "Error getting documents.", usersDocs.exception)
        }
    }

Answer 2

For a web app (JS), this works: db.collection('myCollection').where(firebase.firestore.FieldPath.documentId(), "in", collectionIds).where('status', '==', 2) where collectionIds is an array of Document IDs.