Kotlin Convert Data Class to Map

This is not a straightforward solution, as my use case is slightly differ from the common one.

You will need to add kotlin-reflect.

depedencies {
    implementation "org.jetbrains.kotlin:kotlin-reflect"
}

A lot of the annotation is for Firestore: @IgnoreExtraProperties, @PropertyName, which is not necessary for your use case.

@IgnoreExtraPropertiesdata class Card(    override var id: String? = null,    var modified: Timestamp? = null,    var title: String? = null,    @get:PropertyName(IMAGE_IDS) @set:PropertyName(IMAGE_IDS)    var imageIds: List<String>? = null,    // var place: Place? = null,) : FirestoreData {    companion object {        const val COLLECTION_NAME = "card"        const val MODIFIED = "modified"        const val TITLE = "title"        // const val PLACE = "place"        const val IMAGE_IDS = "image_ids"        fun colRef(firestore: FirebaseFirestore, uid: String) = UserPrivate.colRef(firestore).document(uid).collection(            COLLECTION_NAME        )        fun toObject(doc: DocumentSnapshot): Card? {            val item = doc.toObject<Card>()            item?.id = doc.id            return item        }    }    val isNew: Boolean        get() = id == null    override fun getData(filterNull: Boolean): Map<String, Any?> {        return FirestoreDataUtil.toMap(this, excludeNames = listOf(::COLLECTION_NAME.name), filterNull)    }}

An Interfect class to indicate support to convert data class to map.

interface FirestoreData {    var id: String? // optional    fun getData(filterNull: Boolean = true) : Map<String, Any?>}

Process

Find all the constant public string in companion object to find all the field names (ability to exclude certain fields). With this method, I can add new property to this class where I don't want it to be included in the map.
Get the fields and their value based on these field names.
If object is instance of FirestoreData (like the commented Place field), call getData to get its underlying field.

class FirestoreDataUtil {    companion object {        fun toMap(instance: Any, excludeNames: List<String>, filterNull: Boolean = true) : Map<String, Any?> {            val findMember = instance::class.declaredMemberProperties                .filter{ it.visibility == KVisibility.PUBLIC }                .filterIsInstance<KProperty1<Any, *>>()                .associateBy(                    {                        val annotation = it.getter.findAnnotation<PropertyName>()                        annotation?.value ?: it.name                    },                    { it }                )            var data = instance::class.companionObject!!.declaredMemberProperties                .filter { it.visibility == KVisibility.PUBLIC && it.isConst && it.returnType == String::class.createType() && !excludeNames.contains(it.name) }                .mapNotNull {                    it.getter.call(this).toString()                }                .associateBy(                    { it },                    {                        // raise exception or return null to ignore                        val prop = findMember[it] ?: throw Exception("$it not found")                        val value = prop.get(instance)                        if (value != null) {                            /*                            if (value::class.isData) {                                value::class.memberFunctions.find {  func -> func.name == "toMap" }?.call(value, filterNull) ?: value                            }                             */                            // TODO("Check value is List<FirestoreData> or Map<Any, FirestoreData>")                            if (value is FirestoreData) {                                value.getData(filterNull)                            }                            else value                        } else null                    }                )            if (filterNull) {                data = data.filterValues { it != null }            }            return data        }    }}

Discussions

I didn't use Json or Kotlin Serialization approach because it require each field to be converted into basic type (e.g. Timestamp to epoch milli seconds). I need the value to maintain it's original type.

Originally I use Kotlin Reflect to find the member fields of the data class, but I might accidentally add a new property (e.g. isNew property) which I don't intend for it to end up in the map.