Cannot retrieve a list into Firebase Database

Question

I am currently working on a project with Firebase Database and I cannot retrieve data as a list.

Here a sample of the json I have imported into firebase :

{
  "mood" : [ {
    "date" : 1480625911154,
    "reasons" : "Lorem Ipsum",
    "status" : "Ok",
    "user" : "812c3772-5e3b-4776-8b4c-41ee3047011d"
  }, {
    "date" : 1480626081958,
    "reasons" : "Lorem Ipsum",
    "status" : "Ok",
    "user" : "3fca7432-ac0b-40f8-b898-929df4d3a2e8"
  }, {
    "date" : 1480626567674,
    "reasons" : "Lorem Ipsum",
    "status" : "Ok",
    "user" : "22e9254a-161b-4126-8cd2-7e5ebe03fa6f"
  } ]
}

Into my Android application, I am trying to retrieve data for a specific user with the following query and code (it is in Kotlin) :

FirebaseDatabase.getInstance().getReference("mood").orderByChild("user").equalTo("fa224092-2dfa-49c4-9fb6-c63eccf61a8b").addListenerForSingleValueEvent(object : ValueEventListener
{
  override fun onCancelled(error: DatabaseError)
  {
  }

  override fun onDataChange(snapshot: DataSnapshot)
  {
    moods = snapshot.getValue(object : GenericTypeIndicator<List<Mood>>(){

    })
  }
})

Where moods is declared as : private var moods = emptyList<Mood>() and the Mood class is (it is java) :

@IgnoreExtraProperties
public final class Mood
    implements Serializable
{

  public String user;

  public String status;

  public String reasons;

  public long date;

  public Mood()
  {
    super();
  }

  public Mood(String user, String status, String reasons, long date)
  {
    this.user = user;
    this.status = status;
    this.reasons = reasons;
    this.date = date;
  }

}

Each time, the app crashes on the transformation of the DataSnapshot to a list with the following stacktrace :

12-02 10:20:01.497 22282-22282/com.test.app E/AndroidRuntime: FATAL EXCEPTION: main Process: com.test.app, PID: 22282 com.google.firebase.database.DatabaseException: Expected a List while deserializing, but got a class java.util.HashMap at com.google.android.gms.internal.zzbqi.zza(Unknown Source) at com.google.android.gms.internal.zzbqi.zza(Unknown Source) at com.google.android.gms.internal.zzbqi.zza(Unknown Source) at com.google.firebase.database.DataSnapshot.getValue(Unknown Source) at com.test.app.fragment.StatsFragment$onRetrieveBusinessObjects$1.onDataChange(StatsFragment.kt:46) at com.google.firebase.database.Query$1.onDataChange(Unknown Source) at com.google.android.gms.internal.zzbmz.zza(Unknown Source) at com.google.android.gms.internal.zzbnz.zzYj(Unknown Source) at com.google.android.gms.internal.zzboc$1.run(Unknown Source) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

I do not understand why a HashMap is retrieved... In fact, in previous project, with a similar code, Firebase give me a lsit as expected.

In order to do a comparison, here a code of the previous project (in java).

The json that has been imported into Firebase Database :

{
  "books" : [ {
    "cartoonist" : "Test1",
    "id" : "10",
    "image" : "http://static.fnac-static.com/multimedia/Images/FR/NR/22/e1/7e/8315170/1507-1.jpg",
    "price" : "15.95",
    "publication" : "2016-11-01T00:00:00+01:00",
    "summaryExtract" : "Loremp Ipsum",
    "summaryFull" : "Loremp Ipsum",
    "tome" : "Tome 24 : Le testament de William S"
  }, {
    "cartoonist" : "Riad Sattouf",
    "id" : "1",
    "image" : "http://static.fnac-static.com/multimedia/Images/FR/NR/da/9d/7c/8166874/1507-1.jpg",
    "price" : "20.90",
    "publication" : "2016-10-01T00:00:00+01:00",
    "summaryExtract" : "Loremp Ipsum",
    "summaryFull" : "Loremp Ipsum",
    "tome" : "Tome 3 : L'Arabe du futur"
  }]
}

Here the code (in java) that queries Firebase :

FirebaseDatabase.getInstance().getReference().child("books").addListenerForSingleValueEvent(new ValueEventListener()
{
  @Override
  public void onDataChange(DataSnapshot dataSnapshot)
  {
    final List<Book> books = dataSnapshot.getValue(new GenericTypeIndicator<List<Book>>()
    {
    });
  }

  @Override
  public void onCancelled(DatabaseError databaseError)
  {
  }
});

And the code of the Book class :

@IgnoreExtraProperties
public final class Book
    implements Serializable
{

  public String id;

  public String title;

  public String tome;

  public String cartoonist;

  public String publication;

  public String price;

  public String image;

  public String summaryFull;

  public String summaryExtract;

  public Book()
  {
    super();
  }

  public Book(String id, String cartoonist, String image, String price, String publication, String summaryExtract,
      String summaryFull, String title, String tome)
  {
    this.id = id;
    this.cartoonist = cartoonist;
    this.image = image;
    this.price = price;
    this.publication = publication;
    this.summaryExtract = summaryExtract;
    this.summaryFull = summaryFull;
    this.title = title;
    this.tome = tome;
  }

  @Exclude
  public String getFormattedDate()
  {
    final DateTime dateTime = new DateTime(publication);
    return dateTime.monthOfYear().getAsText(Locale.getDefault()) + " " + dateTime.getYear();
  }

The code works as expected.

Did I miss something with the query in Firebase Database ? Is there a possibility to retrieve my data as a list and not as a hashmap ?

Thank you for your help !

Answer 1

you need to access list of results in onDataChange as follows (this is in Java so will let you convert to Kotlin :) )

                for (DataSnapshot childSnapshot: dataSnapshot.getChildren()) {
                    Book book = childSnapshot.getValue(Book.class);
                }