remote_config_resolve.kt

// This version of the application code was written for the V1 schema.
const val SUPPORTED_SCHEMA_VERSION = 1

private val db = FirebaseFirestore.getInstance()
private val remoteConfig = FirebaseRemoteConfig.getInstance()
private val schemaMappings = mutableMapOf<String,JsonObject>()

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    remoteConfig.fetchAndActivate()
        .addOnCompleteListener(this) { task ->
            if (task.isSuccessful) {
                queryFirestore()
            }
        }
}

private fun queryFirestore() {
    db.collection("people")
        .get()
        .addOnSuccessListener { snapshot ->
            for (doc in snapshot.documents) {
                // getField will transparently map `surname` to the correct field name.
                val surname = doc.getField("surname")
                displayField(surname)
            }            
        }
}

/**
 * Helper method for getting a field value from a snapshot. Transparently
 * resolves the field names to their updated names in newer schema versions.
 */
private fun DocumentSnapshot.getField(field: String): Any? {
    val data = this.data ?: hashMapOf()
    val schemaVersion = data["v"] as? Long ?: 0
    var resolvedField = field
    if (schemaVersion > SUPPORTED_SCHEMA_VERSION) {
        // If the document has a newer schema version, resolve via RemoteConfig.
        val key = "v${SUPPORTED_SCHEMA_VERSION}_to_v${schemaVersion}"
        val mapping = loadMapping(key)
        val temp = mapping.get(field)
        if (temp != null) {
            resolvedField = temp.asString
        }
    }

    Log.d(TAG, "Field resolved: $field --> $resolvedField")
    return data[resolvedField]
}

/**
 * Loads schema mapping instructions from RemoteConfig. Key string has the
 * format: v{sourceVersion}_to_v{targetVersion}.
 */
private fun loadMapping(key: String): JsonObject {
    if (!schemaMappings.containsKey(key)) {
        val parser = JsonParser()
        val mapping = remoteConfig.getString(key)
        if (mapping.isNotEmpty()) {
            schemaMappings[key] = parser.parse(mapping).asJsonObject
        } else {
            schemaMappings[key] = parser.parse("{}").asJsonObject
        }
    }

    return schemaMappings[key]!!
}