Android 从版本 2 到 4 的房间迁移？

Question

I'm trying to migrate a Room database from versions 2 and 3 to 4 like so:

private static final Migration MIGRATION_2_4 = new Migration(2, 4) {
    @Override
    public void migrate(@NonNull @NotNull SupportSQLiteDatabase database) {

        database.execSQL(
                "ALTER TABLE 'market_data' ADD COLUMN 'watch_list_boolean' TEXT NOT NULL DEFAULT '0'");

        database.execSQL("DROP TABLE 'developer_data'");
    }
};

but it's not working, what is wrong here?

Answer 1

The problem, most likely (post your stack trace to help future readers), is that your DB won't be able to perform migrations 2->3 and 3->4

So, your code will only work if your db is upgraded from 2 directly to 4 and will throw an exception (that indicates what migration is missing) if db is upgraded from 2 to 3 or from 3 to 4.

Best practice is to create separate migrations - 2 to 3, and 3 to 4.

Room will know to execute the correct migrations and in the right order (2->3 or 3->4 or 2->3->4):

private static final Migration MIGRATION_2_3 = new Migration(2, 3) {
    @Override
    public void migrate(@NonNull @NotNull SupportSQLiteDatabase database) {
        database.execSQL(
                "ALTER TABLE 'market_data' ADD COLUMN 'watch_list_boolean' TEXT NOT NULL DEFAULT '0'");
    }
};

private static final Migration MIGRATION_3_4 = new Migration(3, 4) {
    @Override
    public void migrate(@NonNull @NotNull SupportSQLiteDatabase database) {
        database.execSQL("DROP TABLE 'developer_data'");
    }
};

Don't forget to update DB version:)

Answer 2

With all due respect, you are taking a conservative approach.

Since the Room database uses Gradle to set its version number, it's very easy to change it.

So, instead of relying on the tools of Gradle and SQLiteDatabase to do this job for you, use the in-memory version of your database and just create the column using plain SQL.