Adding a map to an array on Firestore Android


I’m having a little trouble getting my data to save to a Firestore database. Each document in my “meetings” collection has a list of users (maps), but I cannot seem to add anything or create a new document. I read the documentation, and it said to use FieldValue.arrayUnion(), which I tried:

    private fun addUserToMeeting(user: User) {
        val meetingRef  database.collection("meetings").document(meetingID.toString())

        val userData  hashMapOf(
            "email" to,
            "latitude" to user.latitude,
            "longitude" to user.longitude,
            "token" to user.token,
            "username" to user.username

        meetingRef.update("users", FieldValue.arrayUnion(userData))

This is called anytime an “add user to meeting” button is clicked.

Here’s a picture of my schema: enter image description here

Has anyone got some suggestions on what I might be doing wrong?


If the document doesn’t exist yet, you can’t use update to create it. As its name implies, update can only be used to update an existing document.

To create a new document at a location you control, use set:

meetingRef.set(hashMapOf("users" to FieldValue.arrayUnion(userData)))

Also see the Firebase documentation on setting a document.

Answered By – Frank van Puffelen

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