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Best practice to store object with generic field into a database

I'm implementing an application for private use in Android which request some data from server in JSON format, deselialize it to POJO and need to store objects in a local database. The problem is that Child class contains a generic value, also the value may be primitive type (integer, string, ...) or some custom objects. I tried Realm, but i got NPE. I have read about EAV-Pattern, but I'm not really satisfied with it.
What is the best way to store object containing generic values in a database? I'm not depend on a specific database, but would prefer SQLite.
Simple POJOs of data structure without extends RealmObject:
public class Parent {
public int i;
public Child mChild;
}
A class which contains generic field:
public class Child<T> {
public int i;
public T mValue;
}
A custom object:
public class Value {
public int i;
public String s;
}
An example trying Realm:
Parent p = new Parent();
p.i = 10;
// Child<String> c = new Child<>();
// c.mValue = "I'm a string!";
Child<Value> c = new Child<>();
Value v = new Value();
v.s = "I'm a custom object!";
v.i = 42;
p.mChild = c;
Realm.init(getApplicationContext());
Realm realm = Realm.getDefaultInstance();
realm.executeTransaction(new Realm.Transaction() {
#Override
public void execute(Realm realm) {
Parent parent = realm.copyToRealm(p);
}
});

Query realm data contained on other object

This question is a follow-up question from: Organize Android Realm data in lists
Due to the data returned by the API we use, it's slightly impossible to do an actual query on the realm database. Instead I'm wrapping my ordered data in a RealmList and adding a #PrimaryKey public String id; to it.
So our realm data looks like:
public class ListPhoto extends RealmObject {
#PrimaryKey public String id;
public RealmList<Photo> list; // Photo contains String/int/boolean
}
which makes easy to write to and read from the Realm DB by simply using the API endpoint as the id.
So a typical query on it looks like:
realm.where(ListPhoto.class).equalTo("id", id).findFirstAsync();
This creates a slightly overhead of listening/subscribing to data because now I need to check listUser.isLoaded() use ListUser to addChangeListener/removeChangeListener and ListUser.list as an actual data on my adapter.
So my question is:
Is there a way I can query this realm to receive a RealmResults<Photo>. That way I could easily use this data in RealmRecyclerViewAdapter and use listeners directly on it.
Edit: to further clarify, I would like something like the following (I know this doesn't compile, it's just a pseudo-code on what I would like to achieve).
realm
.where(ListPhoto.class)
.equalTo("id", id)
.findFirstAsync() // get a results of that photo list
.where(Photo.class)
.getField("list")
.findAllAsync(); // get the field "list" into a `RealmResults<Photo>`
edit final code: considering it's not possible ATM to do it directly on queries, my final solution was to simply have an adapter that checks data and subscribe if needed. Code below:
public abstract class RealmAdapter
<T extends RealmModel,
VH extends RecyclerView.ViewHolder>
extends RealmRecyclerViewAdapter<T, VH>
implements RealmChangeListener<RealmModel> {
public RealmAdapter(Context context, OrderedRealmCollection data, RealmObject realmObject) {
super(context, data, true);
if (data == null) {
realmObject.addChangeListener(this);
}
}
#Override public void onChange(RealmModel element) {
RealmList list = null;
try {
// accessing the `getter` from the generated class
// because it can be list of Photo, User, Album, Comment, etc
// but the field name will always be `list` so the generated will always be realmGet$list
list = (RealmList) element.getClass().getMethod("realmGet$list").invoke(element);
} catch (Exception e) {
e.printStackTrace();
}
if (list != null) {
((RealmObject) element).removeChangeListener(this);
updateData(list);
}
}
}

First you query the ListPhoto, because it's async you have to register a listener for the results. Then in that listener you can query the result to get a RealmResult.
Something like this
final ListPhoto listPhoto = realm.where(ListPhoto.class).equalTo("id", id).findFirstAsync();
listPhoto.addChangeListener(new RealmChangeListener<RealmModel>() {
#Override
public void onChange(RealmModel element) {
RealmResults<Photo> photos = listPhoto.getList().where().findAll();
// do stuff with your photo results here.
// unregister the listener.
listPhoto.removeChangeListeners();
}
});
Note that you can actually query a RealmList. That's why we can call listPhoto.getList().where(). The where() just means "return all".
I cannot test it because I don't have your code. You may need to cast the element with ((ListPhoto) element).

I know you said you're not considering the option of using the synchronous API, but I still think it's worth noting that your problem would be solved like so:
RealmResults<Photo> results = realm.where(ListPhoto.class).equalTo("id", id).findFirst()
.getList().where().findAll();
EDIT: To be completely informative though, I cite the docs:
findFirstAsync
public E findFirstAsync()
Similar to findFirst() but runs asynchronously on a worker thread This method is only available from a Looper thread.
Returns: immediately an empty RealmObject.
Trying to access any field on the returned object before it is loaded
will throw an IllegalStateException.
Use RealmObject.isLoaded() to check if the object is fully loaded
or register a listener RealmObject.addChangeListener(io.realm.RealmChangeListener<E>) to be
notified when the query completes.
If no RealmObject was found after
the query completed, the returned RealmObject will have
RealmObject.isLoaded() set to true and RealmObject.isValid() set to
false.
So technically yes, you need to do the following:
private OrderedRealmCollection<Photo> photos = null;
//...
final ListPhoto listPhoto = realm.where(ListPhoto.class).equalTo("id", id).findFirstAsync();
listPhoto.addChangeListener(new RealmChangeListener<ListPhoto>() {
#Override
public void onChange(ListPhoto element) {
if(element.isValid()) {
realmRecyclerViewAdapter.updateData(element.list);
}
listPhoto.removeChangeListeners();
}
}

Firebase won't bind boolean value to field

I'm using Google's firebase-database SDK for Android, v9.0.1. I have my app hooked up to Firebase and can read and write data at various locations.
However, I cannot get a specific boolean field to bind using dataSnapshot.getValue(PingReport.class) and I keep getting an error in my logs that says No setter/field for isUp found on class com.myapp.PingReport when the field clearly exists in my model.
Here's the JSON in the Firebase database:
{
"durationMs": 364,
"isUp": true,
"timestampMillis": 1464916019971
}
and here's the model class:
public class PingReport {
private long durationMs;
private boolean isUp;
private long timestampMillis;
public PingReport() {
// required by Firebase
}
public PingReport(long durationMs, boolean isUp, long timestampMillis) {
this.durationMs = durationMs;
this.isUp = isUp;
this.timestampMillis = timestampMillis;
}
public long getDurationMs() {
return durationMs;
}
public boolean isUp() {
return isUp;
}
public long getTimestampMillis() {
return timestampMillis;
}
}
If I call getDurationMs() or getTimestampMillis() the correct values are returned, but the value returned from isUp() is always false. I have tried different combinations of naming it up and isUp and mUp and adding setters setUp(boolean up) and setIsUp(boolean isUp), but nothing seems to work. The documentation for the Android SDK not very detailed. Is there some trick or detail I'm overlooking?

If your boolean field is named isUp, then the getter must be named isIsUp() or getIsUp(). Alternatively, if you want a getter named isUp, the field name would be up.

Alternatively, you can use the #PropertyName annotation from Firebase Database to handle this. Also, it's better to include setters as well.
Pass a custom Java object, if the class that defines it has a default constructor that takes no arguments and has public getters for the properties to be assigned.
public class PingReport {
private long durationMs;
private boolean isUp;
private long timestampMillis;
public PingReport() {
// required by Firebase
}
public PingReport(long durationMs, boolean isUp, long timestampMillis) {
this.durationMs = durationMs;
this.isUp = isUp;
this.timestampMillis = timestampMillis;
}
public long getDurationMs() {
return durationMs;
}
#PropertyName("isUp")
public boolean isUp() {
return isUp;
}
public long getTimestampMillis() {
return timestampMillis;
}
}

I ran into this problem in Kotlin. I had several boolean values. All of them were set properly except for the one that started with is. To fix this, I made it a custom getter and that fixed the problem:
data class FirebaseEvent(
val description: String? = null,
val disableLogin: Boolean? = null,
val isDVR: Boolean? = null
) {
fun getIsDVR(): Boolean? {
// this is needed to trick Kotlin into using this getter instead of generating its own which breaks firebase
return isDVR
}
}

Don't use "is" with your variable name specifically with Firebase Realtime Database. I also had the same problem, I have to change my variable name from "isUserExist" to "userExist" ! Crippy Firebase!

Your getter for isUp() is not following the normal convention. Have you tried public boolean getIsUp()?

You should check the naming convention in java.
Try to use:
private boolean up;
public boolean isUp() {
return up;
}

if you are using Kotlin then use PropertyName on getter and setter of the variable so that you bypass kotlin generated getters/setters names.
using this annotation on the variable itself doesn't help because it doesnt affect the generated setters and setters.
data class(
#get:PropertyName("isRead")
#set:PropertyName("isRead")
var isRead: Boolean? = null
)
you can check this question for more info about java

Dont use the primitive "boolean" type but use "Boolean" class instead.

How to save and load data in realm

I have a json file and would load to realm file. If I want to update the json file, e.g. notification, how I can do it through realm?
By default notification set to yes, want to update to no. And when restart the file, notification will set to no.
My question is how to to save and load data in realm.
public class Setting extends RealmObject {
#Required
private Boolean Notification;
private RealmList<Translation> Translations;
public Boolean getNotification() {
return Notification;
}
public void setNotification(Boolean notification) {
Notification = notification;
}
public RealmList<Translation> getTranslations() {
return Translations;
}
public void setTranslations(RealmList<Translation> translations) {
Translations = translations;
}
}

I assume you only need one Setting object to be stored in the Realm. So whenever you want to write/read it, always using findFirst to try to get the only Setting object like below:
static Setting getSettingInstance(Realm realm) {
Setting setting = realm.where(Setting.class).findFirst();
if (setting == null) {
// The object doesn't exist.
realm.beginTransaction();
setting = realm.createObject(Setting.class);
setting.setNotification(true); // Make it default to true
realm.commitTransaction();
}
return setting;
}
When you want to change the notification:
Setting setting = getSettingInstance(realm);
realm.beginTransaction();
setting.setNotification(false);
realm.commitTransaction();
I am not quite sure what do you want to do with JSON, but you can have a look with Realm's JSON related functions like this.
About writing to Realm, see doc.

return an object Android

I want to return an object with some things in them.
Here is the declaration;
Object user_det = get_user_det();
Here is the function code:
private Object get_user_det() {
Firebase f_user = new Firebase("https://myapp.firebaseio.com/User/");
f_user.addListenerForSingleValueEvent(new ValueEventListener(){
#Override
public void onDataChange(DataSnapshot snap_user) {
// TODO Auto-generated method stub
Iterable<DataSnapshot> rs = snap_user.getChildren();
Iterator<DataSnapshot> irs = rs.iterator();
long allNum2 = snap_user.getChildrenCount();
int maxNum2 = (int)allNum2;
int count_user = 1;
while(irs.hasNext())
{
if(count_user <= maxNum2)
{
Firebase user_data = new Firebase("https://myapp.firebaseio.com/");
AuthData authData = user_data.getAuth();
Map<String, Object> nPost = (Map<String, Object>) irs.next().getValue();
String db_email = nPost.get("email_addr").toString();
if (authData != null) {
String usr_email = authData.getProviderData().get("email").toString();
if(usr_email.equals(db_email))
{
//NB: I WANT TO ADD THE FOLLOWING INTO THE OBJECT
String disp_name = nPost.get("disp_name").toString();
String real_name = nPost.get("real_name").toString();
}
} else {
System.out.println("Failed");
}
}
count_user++;
}
}
#Override
public void onCancelled(FirebaseError arg0) {
// TODO Auto-generated method stub
}
});
return null; //NB: I NEED TO RETURN THE OBJECT HERE.
}
I want to return the string disp_name and real_name but they are inside the addListenerForSingleValueEvent, so how do I get them out and return it to the function.
I have wrote "NB" in the code where I need help with.
Thanks for your time.

If you want to return an object from your method in java, do it like this:
The Object class:
This contains the structure of your Object, and defines what data will be in it. Also includes methods to easily get the data.
private class myObject {
private String name;
private String realName;
//The constructor, so you can set the data when creating the Object.
public myObject (String disp_name, String real_name) {
name = disp_name;
realName = real_name;
}
//Getter methods, to get the data.
public String getRealName() {return realName;}
public String getDisplayName() {return name;}
}
Your code:
private Object get_user_det() {
myObject o; //Declare it, so it can be returned.
...
String disp_name = nPost.get("disp_name").toString();
String real_name = nPost.get("real_name").toString();
o = new myObject(disp_name, real_name); //create it and set the data.
...
return myobject; //return the new Object with the data.
}
To get the data from the Object:
myObject o = get_user_det(); //Call the metod which return our Object.
String realName = o.getRealName(); //Get the data from the Object.
String displayName = o.getDisplayName;
In your case, it would be much easier to use a String array.
Hope this helps.

It's probably easiest to see what's going on, if you add some printlns to your code:
private Object get_user_det() {
Firebase f_user = new Firebase("https://myapp.firebaseio.com/User/");
System.out.println("Adding listener");
f_user.addListenerForSingleValueEvent(new ValueEventListener(){
#Override
public void onDataChange(DataSnapshot snap_user) {
System.out.println("Data received");
}
#Override
public void onCancelled(FirebaseError arg0) {
// TODO Auto-generated method stub
}
});
System.out.println("Returning");
return null; //NB: I NEED TO RETURN THE OBJECT HERE.
}
If you execute this code, you will see that it logs:
Adding listener
Returning
Data received
Most likely, this is not what you expected. But hopefully, it makes sense if you read my explanation below.
Asynchronous loading
When you register your listener:
f_user.addListenerForSingleValueEvent(new ValueEventListener(){
You tell Firebase to start listening for events. It goes off and starts retrieving the data from the server.
Since retrieving the data may take some time, it does this retrieval asynchronously so that your thread isn't blocked. Once the data is completely retrieved, Firebase calls the onDataChange method in your listener.
Between the time you start listening and the time onDataChange is called, your code continues executing. So there is no way to return data that is loaded asynchronously, because by the time your function returns, the data isn't loaded yet.
Solutions
Disclaimer: I am not an expert at solving this problem in Java, so there may be problems with my solutions. If^H^HWhen you find any, please report them in the comments.
I know of three possible solutions to the problem:
force the code to wait for the data to be returned
return a Future that at some point will contain the data
pass a callback into get_user_det and call that function once the data is available
You will probably be tempted to selected option 1, since it matches most closely with your mental modal of loading data. While this is not necessarily wrong, keep in mind that there is a good reason that the loading is done asynchronously. It might be worth taking the "learning how to deal with asynchronicity" penalty now.
Instead of writing up examples for all solutions, I'll instead refer to some relevant questions:
Retrieving data from firebase returning NULL (an answer that uses approach 3)
Is waiting for return, ok?
Java wait() & notify() vs Android wait() & notify() (a question from a user taking approach 1)

How it works:
Firebase uses reflection to build a JSON tree object to save to the database. When you retrieve this JSON tree, you can cast it back to your original object. Just like serializing and deserializing. This means you do not need to handle the keys and values when trying to "rebuild" your object like you are. It can all be done like so:
YourObject object = (YourObject) dataSnapshot.getValue(YourObject.class);
Notice the YourObject.class in the getValue(). This tells firebase to reflect through this class and find the appropriate accessors with the dataSnapshot.
How to do it
Be sure that your object has:
Accessors Appropriate getters and setters for ALL fields - (or annotated with #JsonIgnore if you wish to not save a particular field)
Empty constructor. Your object must provide a constructor that does not modify itself at all.
What your object should look like:
public class YourObject {
private String displayName;
private String realName;
public YourObject() { /*Empty constructor needed for Firebase */ }
// Accessors
public void setRealName(String realName){
this.realName = realName;
}
public String getRealName(){
return this.realName;
}
public String getDisplayName(){
return this.displayName;
}
public void setDisplayName(String displayName){
this.displayName = displayName;
}
}
Then, in any of the firebase callbacks, you can just cast your DataSnapshot in to your object:
public void onDataChange(DataSnapshot snap_user) {
YourObject object = new Object;
if(snap_user.getValue() != null) {
try {
object = (YourObject) snap_user.getValue(YourObject.class); <-- Improtant!!!
} catch(ClassCastException ex) {
ex.printStackTrace();
}
}
return object;
}
Also
It seems you are retrieving many objects. When doing this, I find it best to use the onChildEventListener then for each of the YourObjects in that node, onChildAdded(DataSnapshot ds, String previousChild); will be called.