Merging Realm object & Gson object

Question

My goal is to persist an object instantiated with GSON to the database with realm.

My problem is the following:

• I can instantiate an object with GSON, so GSON takes care of all the properties, but then it won't be persisted to db

• I can instantiate an object with Realm, but then I have to fill in all the setters for the properties.

In my oncreate, this part is the same for both methods

//delete previous realm file
Realm.deleteRealmFile(this);

//gson instance
Gson gson = new Gson();

//realm instance
realm = Realm.getInstance(this);

** The name of my class is Vacature **

Info: I am aware that the following code needs to be inside:

realm.beginTransaction();

// code here

realm.commitTransaction();

Option 1:

//get class with gson
Vacature vacatureGson = gson.fromJson(jsonString, Vacature.class);

This won't work because you have to instantiate a realm object by using

Class instance = realm.createObject(Class.class); // Create a new object

Option 2:

//get instance with gson
Vacature vacatureGson = gson.fromJson(jsonString, Vacature.class);
//make instance with realm
Vacature realmVacature = realm.createObject(Vacature.class);
realmVacature = vacatureGson;

This won't work because this is not a proper way to copy all the properties from one object to another. If there is however a good way to copy all the properties from the gsonObject to the realmObject, option 2 might work.

Any ideas on how to solve this puzzle?

Answer 1

Emanuele from Realm here.

Right now the solution is suboptimal and is presented in one of the examples: https://github.com/realm/realm-java/blob/master/examples/gridViewExample/src/main/java/io/realm/examples/realmgridview/GridViewExampleActivity.java

We're working to make the experience way smoother!

Answer 2

As it was said in previous answers for serialize RealmObject with Gson the appropriate JsonSerializer should be registered at Gson instance. Below my example of an universal JsonSerializer for Gson that works with getters instead of fields.

Firstly, it should be registered with Gson in order to use like this:

    @NonNull private static Gson createGson() {
            GsonBuilder gsonBuilder = new GsonBuilder().excludeFieldsWithoutExposeAnnotation();
            try {        
                    gsonBuilder.registerTypeAdapter(Class.forName("io.realm.<YourClassName1>RealmProxy"), Serializer.newInstance(<YourClassName1>.class));
                    gsonBuilder.registerTypeAdapter(Class.forName("io.realm.<YourClassName2>RealmProxy"), Serializer.newInstance(<YourClassName2>.class));
                    ...
                } catch (ClassNotFoundException e) {
                    e.printStackTrace();
                }

        return gsonBuilder.create();
    }

And then it is possible to get JSON from any RealmObject-derived (and even created by Realm) class like this:

public class MessageModel extends RealmObject { ... }
String json = createGson().toJson(messageModelInstance);

And you can copy MessageModel like this:

MessageModel src= ...; 
MessageModel dst= ...;

Serializer.newInstance(MessageModel.class).copy(src, dst);

---

And here Serializer

public class Serializer<T> implements JsonSerializer<T> {

    private Class<T> clazz;
    private Serializer(Class<T> clazz) {this.clazz = clazz;}

    @NonNull
    public static <T> Serializer<T> newInstance(Class<T> clazz) {return new Serializer<T>(clazz);}

    @Override
    public JsonElement serialize(T src, Type typeOfSrc, JsonSerializationContext context) {

        final JsonObject jsonObject = new JsonObject();

        Field[] fields = clazz.getDeclaredFields();
        for (Field f : fields) {
            if (f.isAnnotationPresent(Expose.class)) try {

                String name = f.getName();
                String methodName = "get" + name.substring(0, 1).toUpperCase() + name.substring(1);
                Method method = src.getClass().getMethod(methodName);
                Object o = method.invoke(src);

                if (o instanceof String) {
                    jsonObject.addProperty(name, (String) o);
                }
                else if (o instanceof Number) {
                    jsonObject.addProperty(name, (Number) o);
                }
                else if (o instanceof Boolean) {
                    jsonObject.addProperty(name, (Boolean) o);
                }
                else if (o instanceof Character) {
                    jsonObject.addProperty(name, (Character) o);
                }

            } catch (InvocationTargetException e) {
                e.printStackTrace();
            } catch (NoSuchMethodException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        }

        return jsonObject;
    }

    public <T> void copy(T src, T dst) {

        Field[] fields = clazz.getDeclaredFields();
        for (Field f : fields) {
            if (f.isAnnotationPresent(Expose.class)) try {

                String name = f.getName();

                String getterName = "get" + name.substring(0, 1).toUpperCase() + name.substring(1);
                String setterName = "set" + name.substring(0, 1).toUpperCase() + name.substring(1);
                Method getter = clazz.getMethod(getterName);
                Method setter = clazz.getMethod(setterName, f.getType());

                setter.invoke(dst, getter.invoke(src));

            } catch (Exception e) {
                e.printStackTrace();
                checkState(false);
            }
        }
    }
}

Answer 3

This can be achieved with ONE LINE OF CODE.

Get a copy in memory of the managed RealmObject and pass it to Gson

String jsonVacature = new Gson().toJson(realm.copyFromRealm(vacature));

Check this answer for more details and for other solutions.