IssueTracker

Component used: Room Persistence Library
Version used: 1.0.3
Devices/Android versions reproduced on: Emulator and Pixel XL

When I try to go from version 3 to 2 I get the following exception:
01-18 09:33:21.376 8183-8248/org.lds.gliv.dev E/AndroidRuntime: FATAL EXCEPTION: pool-1-thread-1
                                                                Process: org.lds.gliv.dev, PID: 8183
                                                                java.lang.IllegalStateException: A migration from 3 to 2 is necessary. Please provide a Migration in the builder or call fallbackToDestructiveMigration in the builder in which case Room will re-create all of the tables.
                                                                    at android.arch.persistence.room.RoomOpenHelper.onUpgrade(RoomOpenHelper.java:82)
                                                                    at android.arch.persistence.room.RoomOpenHelper.onDowngrade(RoomOpenHelper.java:94)
                                                                    at android.arch.persistence.db.framework.FrameworkSQLiteOpenHelper$OpenHelper.onDowngrade(FrameworkSQLiteOpenHelper.java:128)
                                                                    at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:297)
                                                                    at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:194)
                                                                    at android.arch.persistence.db.framework.FrameworkSQLiteOpenHelper$OpenHelper.getWritableSupportDatabase(FrameworkSQLiteOpenHelper.java:93)
                                                                    at android.arch.persistence.db.framework.FrameworkSQLiteOpenHelper.getWritableDatabase(FrameworkSQLiteOpenHelper.java:54)
                                                                    at android.arch.persistence.room.RoomDatabase.query(RoomDatabase.java:193)
 

Here is my database creation code:
    fun createDatabase(application: Application): UserDatabase {
        return Room.databaseBuilder(application, UserDatabase::class.java, getDatabaseName(userManager.userId))
                .addMigrations(AddStep1_2, DropStep2_1, AddDatum2_3, DropDatum3_2)
                .addCallback(UserDatabase.Callback)
                .build()
    }

Here is my version 2 database class:
@Database(version = 2, entities = [
    /* 1 */ Tag::class, Impression::class, Goal::class,
    /* 2 */ GoalQuestion::class, Question::class, Step::class, StepCompletion::class//,
    /* 3 */ //Datum::class
])
@TypeConverters(ConvertDateTime::class, ConvertStepFrequency::class, ConvertTagId::class, ConvertUUID::class)
abstract class UserDatabase : RoomDatabase() {
...
}

Here are my migrations:

object AddStep1_2 : Migration(1, 2) {
    override fun migrate(db: SupportSQLiteDatabase) {
...
    }
}

object DropStep2_1 : Migration(2, 1) {
    override fun migrate(db: SupportSQLiteDatabase) {
...
    }
}

object AddDatum2_3 : Migration(2, 3) {
    override fun migrate(db: SupportSQLiteDatabase) {
        db.execSQL("CREATE TABLE IF NOT EXISTS `Datum` (`key` TEXT NOT NULL PRIMARY KEY, `value` TEXT)")
    }
}

object DropDatum3_2 : Migration(3, 2) {
    override fun migrate(db: SupportSQLiteDatabase) {
        db.execSQL("DROP TABLE IF EXISTS `Datum`")
    }
}

-------
Looking at the RoomDatabase source code, lines 536-544, the 'if' conditions are the source of the problem, which will only find forward migrations, even though you have an 'upgrade' variable just above which is false for reverse migrations:

                for (int i = firstIndex; i != lastIndex; i += searchDirection) {
                    int targetVersion = targetNodes.keyAt(i);
                    if (targetVersion <= end && targetVersion > start) {
                        result.add(targetNodes.valueAt(i));
                        start = targetVersion;
                        found = true;
                        break;
                    }
                }