How to select from multiple databases in Android Room(How to attach databases)

Question

As you know we can select from multiple databases using attach command like this:

String path = DBHelper.getDatabasePath(context);
String sql = \"ATTACH DATA         


        

Answer 1

While building room database using the DatabaseBuilder class, you have an option to register a callback which has methods that'd be called everytime your database is opened:

 /**
         * Called when the database has been opened.
         *
         * @param db The database.
         */
        public void onOpen(@NonNull SupportSQLiteDatabase db) {
        }

One option is to register this callback and ATTACH your other database using the db parameter. Then in dao, you can have queries which reference the other table.