Jetpack Room入门系列：（五）数据库版本升级、数据迁移

爱被打了一巴掌 2022-11-19 11:16 264阅读 0赞

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3ZpdGF2aXZh_size_16_color_FFFFFF_t_70]  
[Jetpack Room入门系列：（一）基本介绍][Jetpack Room]  
[Jetpack Room入门系列：（二）使用DAO读写数据库][Jetpack Room_DAO]  
[Jetpack Room入门系列：（三）实体/数据表关系][Jetpack Room 1]  
[Jetpack Room入门系列：（四）内部实现原理][Jetpack Room 2]  
[Jetpack Room入门系列：（五）数据库版本升级、数据迁移][Jetpack Room 3]  
[Jetpack Room入门系列：（六）配合LiveData等三方库的使用][Jetpack Room_LiveData]

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当数据库的表结构发生变化时，我们需要通过数据库迁移（Migrations）升级表结构，避免数据丢失。

例如，我们想要为User表增加age字段

| uid | first_name | last_name |

↓↓

| uid | first_name | last_name | age |

数据迁移需要使用Migration类：

val MIGRATION_1_2 = object : Migration(1, 2) { 
        override fun migrate(database: SupportSQLiteDatabase) { 
            database.execSQL("ALTER TABLE users ADD COLUMN age INTEGER")
        }
    }

Migration通过`startVersion`和`endVersion`表明当前是哪个版本间的迁移，然后在运行时，按照版本顺序调用各Migration，最终迁移到最新的Version

创建Database时设置Migration：

Room.databaseBuilder(
        applicationContext, 
        UserDatabase::class.java, 
        "users-db"
    ).addMigrations(MIGRATION_1_2)
    .build()

# 迁移失效 #

--------------------

迁移中如果找不到对应版的Migration，会抛出`IllegalStateException`：

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.

可以添加降级处理，避免crash：

Room.databaseBuilder(
        applicationContext, 
        UserDatabase::class.java, 
        "users-db"
    ).fallbackToDestructiveMigration()
    .build()

*  **fallbackToDestructiveMigration**：迁移失败时，重建数据库表
 *  **fallbackToDestructiveMigrationFrom**：迁移失败时，基于某版本重建数据库表
 *  **fallbackToDestructiveMigrationOnDowngrade**：迁移失败，数据库表降级到上一个正常版本

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[Jetpack Room]: https://blog.csdn.net/vitaviva/article/details/112596188
[Jetpack Room_DAO]: https://blog.csdn.net/vitaviva/article/details/112721321
[Jetpack Room 1]: https://blog.csdn.net/vitaviva/article/details/112725459
[Jetpack Room 2]: https://blog.csdn.net/vitaviva/article/details/112727555
[Jetpack Room 3]: https://blog.csdn.net/vitaviva/article/details/112744536
[Jetpack Room_LiveData]: https://blog.csdn.net/vitaviva/article/details/112746243