Google Cloud Functions - Realtime Database Trigger - how to deserialize data JSON to POJO? - kotlin

Question

As described on the Google Cloud Functions docs, it is possible to trigger a Function based on Firebase Realtime Database events (write/create/update/delete).
The following docs sample explains how to get the delta snapshot.
public class FirebaseRtdb implements RawBackgroundFunction {
private static final Logger logger = Logger.getLogger(FirebaseRtdb.class.getName());
// Use GSON (https://github.com/google/gson) to parse JSON content.
private static final Gson gson = new Gson();
#Override
public void accept(String json, Context context) {
logger.info("Function triggered by change to: " + context.resource());
JsonObject body = gson.fromJson(json, JsonObject.class);
boolean isAdmin = false;
if (body != null && body.has("auth")) {
JsonObject authObj = body.getAsJsonObject("auth");
isAdmin = authObj.has("admin") && authObj.get("admin").getAsBoolean();
}
logger.info("Admin?: " + isAdmin);
if (body != null && body.has("delta")) {
logger.info("Delta:");
logger.info(body.get("delta").toString());
}
}
}
The sample works perfectly but the question is: How can I deserialize this delta to a POJO?
I tried:
val mObject = gson.fromJson(body.get("delta").toString(), MyCustomObject::class.java)
But I am getting:
com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT
As far as I know it is because MyObject class has a List<T> field, and Firebase Database always convert Lists to Maps with integer keys.
I preferably do not want to change every List<T> to Map<Int,T>, because I have a lot of classes :(
Thanks in advance!

Answer 1

So, here is what I ended up doing (maybe not the best solution!):
1) Create a custom Json Deserializer for Firebase-coming lists:
class ListFirebaseDeserializer<T> : JsonDeserializer<ArrayList<T>> {
override fun deserialize(json: JsonElement?, typeOfT: Type?, context: JsonDeserializationContext?): ArrayList<T> {
val result = ArrayList<T>()
val typeOfElement = (typeOfT as ParameterizedType).actualTypeArguments[0]
json?.let {
json.asJsonObject.entrySet().forEach {
entry->
result.add(Gson().fromJson(entry.value, typeOfElement))
}
}
return result
}
}
This takes the lists that Firebase turned into maps and convert it back to actual lists.
2) Annotate every list in my POJO with #JsonAdapter(ListFirebaseDeserializer::class), for instance:
class MyCustomObject {
#JsonAdapter(ListFirebaseDeserializer::class)
var myPaymentList = ArrayList<Payment>()
}
It could be a pain if you already have lots of lists to annotate, but it is better than having to use maps instead.
Hope it helps!