How to query a Firestore collection using a property from a step deeper with Kotlin?

Question

I have a Cloud Firestore database for transportations:

• Transportations {collection}
  • TransportationId {document}
    • TransportationName: "Private Transaportation"
    • Cars {collection}
      • CarId {document}
        • CarName: "Taxi"
      • CarId {document}
        • CarName: "Private Taxi"

In Cars collection I have over 100 cars (documents). I cannot find in the documentation nor on stackoverflow how to get all the transportation names in which the CarName name has a specific particular value. How to solve this?

val Taxi = db
    .collection("Transportations").document("TransportationId")
    .collection("Cars").whereEqualTo("CarName", "Private Taxi")

Thanks!

Answer 1

The short answer is that you cannot achieve this in Firestore. You cannot query your database for all the cars regardless of which transportation contained them.

But in such cases, there is a workaround in which you should change your database structure a little bit by creating a new top-level collection named cars in which you should add all your car objects. Beheath each car object you'll also should add the desired transportation as a property. In this way you'll be able to query the database according to your needes.

Your database structure should look like this:

Firestore-root
   |
   --- cars (collection)
        |
        --- carId (document)
              |
              --- transportation: "Private Taxi"
              |
              --- //other car details