为什么 Room 在迁移后删除我的数据库

Question

I want create new table in database. After migration app removed all items in database. Why? I haven't used fallbackToDestructiveMigration()

My code:

public class DatabaseClient {

private static DatabaseClient mInstance;
private DatabaseApp databaseApp;

private DatabaseClient(Context context) {
    appDatabase = Room.databaseBuilder(context, DatabaseApp.class, "DB_NAME").build();
    appDatabase = Room.databaseBuilder(context, DatabaseApp.class, "DB_NAME").addMigrations(MIGRATION_1_2).build();
}

public static synchronized DatabaseClient getInstance(Context mCtx) {
    if (mInstance == null)
        mInstance = new DatabaseClient(mCtx);
    return mInstance;
}

public AppDatabase getAppDatabase() {
    return appDatabase;
}

static final Migration MIGRATION_1_2 = new Migration(1, 2) {
    @Override
    public void migrate(@NonNull final SupportSQLiteDatabase database) {
        database.execSQL("CREATE TABLE `MarkRead` (`Id` INTEGER, `StudentId` TEXT," +
                "`StudentName` TEXT, `MarkId` INTEGER,`Mark` TEXT, PRIMARY KEY(`Id`))");
    }
};
}

What am I doing wrong?

Answer 1

This is because your old data has not been moved to the new table.

You will have to perform the following steps:

1. Create a temp table that you want to eventually CREATE.

CREATE TABLE TEMP (... put your columns here)

2. Fetch the results from your existing table by writing a "SELECT * " query.

3. Insert the results of the previous query into the Temp table.

INSERT INTO TEMP SELECT .... FROM ORIGINAL

4. Drop your existing table.

DROP TABLE ORIGINAL

5. Rename your temp table with the name of the existing table.

ALTER TABLE TEMP RENAME TO ORIGINAL

Now you will have a new table with the same name and the data as well.