Firebase Firestore toObject() with Kotlin

Question

I try to use Firebase Firestore in a Kotlin project. Everything is going fine except when I want to instantiate an object with DocumentSnapshot.toObject(Class valueType).

Here is the code :

FirebaseFirestore
    .getInstance()
    .collection("myObjects")
    .addSnapshotListener(this,
    { querySnapshot: QuerySnapshot?, e: FirebaseFirestoreException? ->

        for (document in querySnapshot.documents) {

            val myObject = document.toObject(MyObject::class.java)

            Log.e(TAG,document.data.get("foo")) // Print : "foo"
            Log.e(TAG, myObject.foo) // Print : ""
        }
    }
})

As you can see, when I use documentChange.document.toObject(MyObject::class.java), my object is instantiated but the inner fields are not set. I know that Firestore needs the model to have an empty constructor. So here is the model :

class MyObject {

    var foo: String = ""

    constructor(){}

}

Can somebody tell me what I'm doing wrong?

Thank you

Gui Silva · Accepted Answer · 2017-10-08 21:55:25Z

33

You forgot to include the public constructor with arguments, or you can also just use a data class with default values, it should be enough:

data class MyObject(var foo: String = "")

edited Oct 8, 2017 at 21:55

answered Oct 8, 2017 at 21:42

Gui Silva

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4 Comments

Sam Stern Over a year ago

Using a data class is likely the best option since it creates publicly named getters/setters for each field. The Firestore data mapper looks for these (so to instantiate foo it needs a public setFoo())

James Moore Over a year ago

And the failure mode for not having a default constructor is almost useless - you'll get "java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.lang.reflect.Constructor.newInstance(java.lang.Object[])' on a null object reference.

Naetmul Over a year ago

All Kotlin classes generate publicly named getters and setters for each public var field and publicly named getters for each public val field. Not only data classes.

Naetmul Over a year ago

To avoid the error, just create a data class and call the primary constructor in the empty constructor like constructor() : this("", null, 0)

vovahost · Accepted Answer · 2018-02-18 23:03:28Z

11

In my case I was getting a NullPointerException because there wasn't a default constructor. Using a data class with default values fixed the error.

data class Message(
        val messageId : String = "",
        val userId : String = "",
        val userName : String = "",
        val text : String = "",
        val imageUrl : String? = null,
        val date : String = ""
)

answered Feb 18, 2018 at 23:03

vovahost

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2 Comments

Bitwise DEVS Over a year ago

What would be the difference if we omit the default values? Also what will happen when the property of non-nullable data class does not exist in document? Thanks

vovahost Over a year ago

A default empty constructor will NOT be generated if you specify all the default values as shown above. Just set the properties to something that makes sense, in my case it was empty string for String type for example.

Swati · Accepted Answer · 2019-04-30 06:07:16Z

0

class MyObject {

    lateinit var foo: String

    constructor(foo:String) {
        this.foo = foo
    }

    constructor()

}

edited Apr 30, 2019 at 6:07

Swati

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answered Apr 30, 2019 at 5:21

Gomax Lo

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