Room database schema update

Question

My application is in release mode and I am using room database and my previous database version was 2 with fallback to destructive migration enabled.

@Database(entities = {
        User.class,ApplicationSetting.class},
        version = 2,
        exportSchema = false)
abstract public class DatabaseContext extends RoomDatabase {

    private static final Object sLock = new Object();
    private static DatabaseContext INSTANCE;
    public static String DATABASE_NAME = AppConstants.DATABASE_NAME;

    public static DatabaseContext getInstance(Context context) {
        synchronized (sLock) {
            if (INSTANCE == null) {
                INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                        DatabaseContext.class, DATABASE_NAME)
                        .fallbackToDestructiveMigration()
                        .build();
            }
            return INSTANCE;
        }
    }

I have added a new column in table and change version to 3. Now I want to provide migration from version 2 to version 3 so that data not lost. But I am confused because in version 2 I have enabled fallback destructive migration, and now in version 3 I want to keep user data and remove fallback destruction.

How can I achieve this?

Answer 1

When using fallbackToDestructiveMigration, it only destroys (drops the tables and recreates them) if there is not a defined migration for the migration so you could add a 2 to 3 Migration.

That is, if a Migration is provided then it is used and bypasses fallback/destruction.

An alternative, which is recommended, is to use fallbacktodestructivemigrationfrom, this can be used to define specific missing migrations where fallback is to be applied.

e.g. you could use .fallbackToDestructiveMigrationFrom(1,7)

• 1 to allow destructive from 1 to 2
• 7 to allow destructive from 7 to 8 (added just to show that multiple start versions can be provided)
• other's 2 to 3, 3 to 4 etc will need a Migration.
• Note that this is more specific, so if a Migration for 1 to 2 or 7 to 8 is provided an exception will result.