How to add existing objects on a new class on Realm Migration in Android

Question

I have an app on production, so changes has to be applied with a RealmMigration

I've looked the documentation and this example but I didn't find how to do the following.

In current version I have items of type Foo that has a boolean property called favorite. Now I want to generalize that and create user custom Foo lists, so the users will be able to create their custom lists and add as many objects as they want. I want to implement this with a new class called UserFooList with basically name and a RealmList<Foo>of elements.

In migration process I'm creating this new class with its fields.

That's easy, but here comes the difficult part:

I want to add all previous Foo items flagged with favorite to a new UserFooList and then remove the now unused Foo's field favorite

---

Some code to help to clarify:

Current class

public class Foo extends RealmObject{

    private String title;
    private boolean favorite;

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public boolean isFavorite() {
        return favorite;
    }

    public void setFavorite(boolean favorite) {
        this.favorite = favorite;
    }
}

Changed class

public class Foo extends RealmObject{

    //favorite field will be removed
    private String title;

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

New class

public class UserFooList extends RealmObject{

    private String name;
    private RealmList<Foo> items;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public RealmList<Foo> getItems() {
        return items;
    }

    public void setItems(RealmList<Foo> items) {
        this.items = items;
    }
}

I want to insert a UserFooList instance and populate it with:

• name: "favorites"
• items: all existing Foo instances with favorite == true

And I want to do it during Migration because in that way I will be able to remove favorite field after inserting all elements in the new created list.

Atmospheric

If you add the previous model and the new model you want, I'll be able to know what you want to call your fields – Potaufeu

@Potaufeu thanks for your comment, I edited my answer to try to explain it better with some example code. – Atmospheric

Answer 1

Rely on the power of the DynamicRealm API.

public class MyMigration implements Realm.Migration {    
    @Override
    public void migrate(DynamicRealm realm, long oldVersion, long newVersion) {
        RealmSchema schema = realm.getSchema();
        if(oldVersion == 0) {
            RealmObjectSchema foo = schema.get("Foo");
            RealmObjectSchema userFooList = schema.create("UserFooList");
            userFooList.addField("name", String.class);
            userFooList.addRealmListField("items", foo);

            DynamicRealmObject userList = realm.createObject("UserFooList");
            userList.setString("name", "favorites");
            RealmList<DynamicRealmObject> listItems = userList.getList("items");
            RealmResults<DynamicRealmObject> favoriteFoos = realm.where("Foo").equalTo("favorite", true).findAll();
            for(DynamicRealmObject fooObj: favoriteFoos) {
                listItems.add(fooObj);
            }
            foo.removeField("favorite");
            oldVersion++;
        }
    }

    @Override public boolean equals(Object object) { 
        return object != null && object instanceof MyMigration;
    }

    @Override public int hashCode() {
        return MyMigration.class.hashCode();
    }
}

Potaufeu

Interestingly, I don't think I've had to answer a migration question with this level of api usage, so that's interesting after these past 3 years. hah – Potaufeu

Thanks. Yes, it's a little bit tricky. I'm glad this awesome api allows to do it as easy as that. – Atmospheric

@Potaufeu , Please check stackoverflow.com/questions/59855380/… about realm migration – Superorder