Employing Migrations

Simply creating a Migration as a static field somewhere is necessary but not sufficient to have Room know about performing the migration. Instead, you need to use the addMigrations() method on RoomDatabase.Builder to teach Room about your Migration objects. addMigrations() accepts a varargs, and so you can pass in one or several Migration objects as needed.

package com.commonsware.android.room;

import android.arch.persistence.room.Database;
import android.arch.persistence.room.Room;
import android.arch.persistence.room.RoomDatabase;
import android.content.Context;

@Database(
  entities={Trip.class, Lodging.class, Flight.class},
  version=2
)
abstract class TripDatabase extends RoomDatabase {
  abstract TripStore tripStore();

  private static final String DB_NAME="trips.db";
  private static volatile TripDatabase INSTANCE=null;

  synchronized static TripDatabase get(Context ctxt) {
    if (INSTANCE==null) {
      INSTANCE=create(ctxt, false);
    }

    return(INSTANCE);
  }

  static TripDatabase create(Context ctxt, boolean memoryOnly) {
    return(create(ctxt, DB_NAME, memoryOnly));
  }

  static TripDatabase create(Context ctxt, String name, boolean memoryOnly) {
    RoomDatabase.Builder<TripDatabase> b;

    if (memoryOnly) {
      b=Room.inMemoryDatabaseBuilder(ctxt.getApplicationContext(),
        TripDatabase.class);
    }
    else {
      b=Room.databaseBuilder(ctxt.getApplicationContext(), TripDatabase.class,
        name);
    }

    return(b.addMigrations(Migrations.FROM_1_TO_2).build());
  }
}

(from Trips/RoomMigrations/app/src/main/java/com/commonsware/android/room/TripDatabase.java)

Here, we teach the RoomDatabase.Builder about the FROM_1_TO_2 Migration. In this sample project, the migrations are implemented in a separate Migrations class, though you are welcome to have them directly in your RoomDatabase class or wherever makes sense for you.

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