Android 请在生成器中提供迁移，或在生成器中调用Fallback ToDestructiveMigration，在这种情况下，Room将重新创建所有表

我正在使用RxJava2的房间。我在表中添加了一列，因此我正在迁移到新版本。我已将数据库版本更改为2

下面是我的迁移代码

static final Migration MIGRATION_1_2 = new Migration(1, 2) {
    @Override
    public void migrate(SupportSQLiteDatabase database) {
        database.execSQL("ALTER TABLE users "
        +"ADD COLUMN address String");

    }
};

AppDatabase db = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, DB_NAME)
.addMigrations(MIGRATION_1_2)
.build();

https://github.com/alahammad/RoomSample

如果您想查看完整的代码， 我在Github上引用了这个示例，它没有迁移代码

static final Migration MIGRATION_1_2 = new Migration(1, 2) {
    @Override
    public void migrate(SupportSQLiteDatabase database) {
        database.execSQL("ALTER TABLE users "
        +"ADD COLUMN address String");

    }
};

AppDatabase db = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, DB_NAME)
.addMigrations(MIGRATION_1_2)
.build();

https://github.com/alahammad/RoomSample

我正在遵循文档中描述的步骤，但我的应用程序仍然崩溃

错误日志

Process: demo.karaoke.sensibol.com.roomrajava2, PID: 13655
    io.reactivex.exceptions.OnErrorNotImplementedException: A migration from 1 to 2 is necessary. Please provide a Migration in the builder or call fallbackToDestructiveMigration in the builder in which case Room will re-create all of the tables.
        at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:704)
        at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:701)
        at io.reactivex.internal.operators.maybe.MaybeCallbackObserver.onError(MaybeCallbackObserver.java:83)
        at io.reactivex.internal.operators.maybe.MaybeObserveOn$ObserveOnMaybeObserver.run(MaybeObserveOn.java:99)
        at io.reactivex.android.schedulers.HandlerScheduler$ScheduledRunnable.run(HandlerScheduler.java:109)
        at android.os.Handler.handleCallback(Handler.java:815)
        at android.os.Handler.dispatchMessage(Handler.java:104)
        at android.os.Looper.loop(Looper.java:194)
        at android.app.ActivityThread.main(ActivityThread.java:5643)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:960)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
     Caused by: java.lang.IllegalStateException: A migration from 1 to 2 is necessary. Please provide a Migration in the builder or call fallbackToDestructiveMigration in the builder in which case Room will re-create all of the tables.
        at android.arch.persistence.room.RoomOpenHelper.onUpgrade(RoomOpenHelper.java:82)
        at android.arch.persistence.db.framework.FrameworkSQLiteOpenHelper$OpenHelper.onUpgrade(FrameworkSQLiteOpenHelper.java:118)
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:256)
        at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
        at android.arch.persistence.db.framework.FrameworkSQLiteOpenHelper$OpenHelper.getWritableSupportDatabase(FrameworkSQLiteOpenHelper.java:93)
        at android.arch.persistence.db.framework.FrameworkSQLiteOpenHelper.getWritableDatabase(FrameworkSQLiteOpenHelper.java:54)
        at android.arch.persistence.room.RoomDatabase.query(RoomDatabase.java:193)
        at demo.karaoke.sensibol.com.roomrajava2.UserDao_Impl$4.call(UserDao_Impl.java:137)
        at demo.karaoke.sensibol.com.roomrajava2.UserDao_Impl$4.call(UserDao_Impl.java:135)
        at io.reactivex.internal.operators.maybe.MaybeFromCallable.subscribeActual(MaybeFromCallable.java:46)
        at io.reactivex.Maybe.subscribe(Maybe.java:3749)
        at io.reactivex.internal.operators.maybe.MaybeSubscribeOn$SubscribeTask.run(MaybeSubscribeOn.java:54)
        at io.reactivex.Scheduler$DisposeTask.run(Scheduler.java:452)
        at io.reactivex.internal.schedulers.ScheduledRunnable.run(ScheduledRunnable.java:66)
        at io.reactivex.internal.schedulers.ScheduledRunnable.call(ScheduledRunnable.java:57)
        at java.util.concurrent.FutureTask.run(FutureTask.java:237)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:152)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:265)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
        at java.lang.Thread.run(Thread.java:818)

---

我从GitHub运行了你的应用程序，并从版本1迁移到版本2。结果发现SQL查询中有一个错误。应该是：

static final Migration MIGRATION_1_2 = new Migration(1, 2) {
    @Override
    public void migrate(SupportSQLiteDatabase database) {
        database.execSQL("ALTER TABLE users "
        +"ADD COLUMN address TEXT");

    }
};

字符串>文本

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最好将Room的数据库实例设置为单例，并且只在Repo/CacheManager类中使用它。请检查完整代码更改的要点-

在生成（）之前添加“.fallbackToDestructiveMigration（）”，

如果您不想提供迁移，并且特别希望在升级版本时清除数据库，请在数据库生成器中调用

fallbackToDestructiveMigration

database = Room.databaseBuilder(context.getApplicationContext(),
                    UsersDatabase.class, "Sample.db")
            .fallbackToDestructiveMigration()
            .build();

---

如果可以接受丢失现有数据，请在创建数据库时调用

fallbackToDestructiveMigration（）

builder方法

示例：

db = Room.databaseBuilder(getApplicationContext(),
                    User.class, "DB_Name")
            .fallbackToDestructiveMigration()
            .build();

在生成（）之前添加“.fallbackToDestructiveMigration（）”，

---

同时也增加了你的版本号。希望有人能找到它。这里。

您可以添加错误吗logs@AmjadKhan我已经添加了错误日志，请看一看您是否已将@Database（entities={}，version=1）注释添加到AppDatabase类中？您是否在手机上安装了版本1，然后使用migration和version=2更新代码并尝试安装？@lomza是的，我这样做了。对于Version1@Database（entities={User.class}，Version=1），我在手机上安装了应用程序，然后添加了一列并更改了@Database（entities={User.class}，Version=2），然后再次在手机上安装了应用程序。有一篇关于迁移的好文章-也许你会在那里找到解决方案……谢谢你的回答。让我查一查，然后给你回电话。你抽出时间检查我的代码对我来说真的很重要。欣赏它即使我写的是字符串而不是文本，它也能工作。问题出在我的Singleton类中，您在中提供了该类，但前提是您希望用户释放所有保存的数据