可以使用此代码附加另一个数据库

Can use this code for attach another database

@Database(entities = {Book.class, User.class}, version = 1)
public abstract class LoanDatabase extends RoomDatabase {

    public abstract UserDao userDao();

    public abstract BookDao bookDao();

    private static LoanDatabase INSTANCE;

    public static LoanDatabase getInstance(Context context,final String attachDatabaseName) {
        if (INSTANCE == null) {{
            INSTANCE = Room.databaseBuilder(context,
                    LoanDatabase.class, "LoanDatabase").addCallback(new Callback() {
                @Override
                public void onOpen(@NonNull SupportSQLiteDatabase db) {
                    attach(attachDatabaseName,"/data/data/com.test.roomwithdagger/databases/");
                    super.onOpen(db);
                }
            })
                    .build();
        }}
        return INSTANCE;
    }

    private static void attach(final String databaseName, final String databasePath) {
        String sql = "ATTACH DATABASE '" + databasePath + databaseName
                + "' AS \"" + databaseName + "\";";
        INSTANCE. mDatabase.execSQL(sql);
    }
}

public void attachDatabase(String databaseName,String databasePath){

    String sql = "ATTACH DATABASE '" + databasePath + "/" + databaseName
            + ".db' AS \"" + databaseName + "\";";
    INSTANCE.mDatabase.execSQL(sql);
}
}

在Dao界面中，使用@SkipQueryVerification这样的跳过查询验证.

In Dao interface use @SkipQueryVerification for skip query verification like this.

@Dao
public interface BookDao {

...

    @SkipQueryVerification
    @Query("SELECT * FROM main.Book b INNER JOIN LoanDatabase1.Loan l on b.Id=l.BookId where b.Id=:bookId")
    Book getBookAndLoan(int bookId);
...

}

使用:

LoanDatabase db = LoanDatabase.getInstance(this,"LoanDatabase1")

Book book= db.bookDao().getBookAndLoan(1)