[英]How to select from multiple databases in Android Room(How to attach databases)
如您所知,我們可以使用attach命令從多個數據庫中選擇,如下所示:
String path = DBHelper.getDatabasePath(context);
String sql = "ATTACH DATABASE '" + path + "/" + dbname.toString()
+ ".db' AS \"" + dbname.toString() + "\";";
db.execSQL(sql);
然后通過使用Cursor,我們可以從中進行選擇。
通過使用Android Room,我該怎么做? 是否有任何附件或類似命令來執行此操作?
可以使用此代碼附加另一個數據庫
@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
進行跳過查詢驗證。
@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)
在使用DatabaseBuilder類構建會議室數據庫時,您可以選擇注冊一個回調 ,該回調具有每次打開數據庫時都會調用的方法:
/**
* Called when the database has been opened.
*
* @param db The database.
*/
public void onOpen(@NonNull SupportSQLiteDatabase db) {
}
一種選擇是使用db
參數注冊此回調並使用其他數據庫進行ATTACH
。 然后在dao中,您可以擁有引用其他表的查詢。
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