Firebase – Can't convert object of type java.lang.String to type com.example.myapplication.Message

I’d like to implement RecyclerView, along with Firebase to my app. I have a simple structure of the firebase:

The keys, under the Message node, are randomly generated by using .push() on reference. The values are just some dummy messages.

Model

data class Message (val message: String? = null)

MainActivity

    private lateinit var mAdapter: MessageAdapter
    ...

    myRef = FirebaseDatabase.getInstance().getReference("Message")

    //Set database query
    val messages = FirebaseRecyclerOptions.Builder<Message>()
        .setQuery(myRef, Message::class.java)
        .build()

    mAdapter = MessageAdapter(messages)

    //Read data
    val rv = findViewById<RecyclerView>(R.id.messages_rv)
    rv.layoutManager = LinearLayoutManager(this)
    rv.adapter = mAdapter

Adapter

    class MessageAdapter(messages: FirebaseRecyclerOptions<Message>) :
    FirebaseRecyclerAdapter<Message, MessageAdapter.ViewHolder>(messages) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        return ViewHolder(
            LayoutInflater.from(parent.context).inflate(R.layout.message_row, parent, false)
        )
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int, model: Message) {
        holder.message.text = model.message
    }

    // Make a textview variable, and initiate it in the init block, which will be called first after creating the ViewHolder object
     class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        val message: TextView = view.findViewById(R.id.message_tv)
    }
}

Unfortunately, my app crash with this error:

com.google.firebase.database.DatabaseException: Can't convert object of type java.lang.String to type com.example.myapplication.Message

I’ve tried to make a different references to the database, but none of them work. How I may solve it?

>Solution :

When you’re using the following reference:

myRef = FirebaseDatabase.getInstance().getReference("Message")

And you pass it to the setQuery() method:

.setQuery(myRef, Message::class.java)

Your adapter expects to render on the screen Message objects. However, under the Message node, there are no such objects but only strings. If you want to map a node into an object of type Message, then you should consider using a schema that looks like this:

db
|
--- Message
     |
     --- $pushedId
     |     |
     |     --- message: "asd"
     |
     --- $pushedId
           |
           --- message: "asd13"

This means that the actual message should exist under a pushed ID.