Room数据库迁移

       App迭代过程中，难免需要对数据库进行更新，有可能是字段的增加，修改或删除，也有可能需要新建一张表，这就涉及到数据库的迁移Migrate,最简单粗暴的方法是在Room的配置项中添加fallbackToDestructiveMigration()方法，如下

 Room.databaseBuilder(context.applicationContext,
                AppDatabase::class.java, "exercise.db") 
        .fallbackToDestructiveMigration()    //添加此方法
        .build()

       为什么说是简单粗暴呢，因为它是直接删除之前的数据库，重新再创建新的数据库，这在大部分情况下绝对是灾难性的做法，所以更好的做法是addMigrations(Migration... migrations)，具体以下步骤：

一、version升级+1 （我这边是3 -> 4）

@Database(entities = [HistoryEntity::class,CollectEntity::class], version = 4)
abstract class AppDatabase: RoomDatabase() {
   //code
}

二、自定义Migration，如下三种情况：

1.增加一个字段update_time

private val MIGRATION_1_2: Migration = object : Migration(1, 2) {
            override fun migrate(database: SupportSQLiteDatabase) {
                database.execSQL("ALTER TABLE history ADD COLUMN update_time INTEGER NOT NULL DEFAULT 0")
            }
        }

2.创建一个新表collect

     private val MIGRATION_2_3: Migration = object : Migration(2, 3) {
            override fun migrate(database: SupportSQLiteDatabase) {
                database.execSQL(
                    "CREATE TABLE collect (collect_id INTEGER NOT NULL DEFAULT 0,line_name_collect TEXT NOT NULL DEFAULT '', upper_or_down_collect TEXT NOT NULL DEFAULT '', PRIMARY KEY(collect_id))")
            }
        }

3.更改一个字段类型

 private val MIGRATION_4_3: Migration = object : Migration(3, 4) {
            override fun migrate(database: SupportSQLiteDatabase) {
                //创建一个备份的新表
                database.execSQL(
                    "CREATE TABLE collect_new_one (collect_id INTEGER NOT NULL DEFAULT 0,line_name_collect TEXT NOT NULL DEFAULT '', upper_or_down_collect TEXT NOT NULL DEFAULT '', PRIMARY KEY(collect_id))")
                // 把旧表的数据拷贝到新表
                database.execSQL(
                    "INSERT INTO collect_new_one (collect_id , line_name_collect, upper_or_down_collect) SELECT collect_id, line_name_collect, upper_or_down_collect FROM collect")
                // 删除掉旧表
                database.execSQL("DROP TABLE collect")
                // 重命名新表（完成谋权篡位）
                database.execSQL("ALTER TABLE collect_new_one RENAME TO collect")
            }
        }

三、将自定义好的Migration加入配置

 Room.databaseBuilder(
                context.applicationContext,
                AppDatabase::class.java, "exercise.db"
            )   .addMigrations(MIGRATION_1_2,MIGRATION_2_3,MIGRATION_3_4)
                .build()

       这就是完整的Room数据库迁移，但是这当中可能会碰到几个异常报错，比如
Room Database Migration doesnt properly handle ALTER TABLE migration
解决方法可以定位到我第一个例子（增加一个字段）的sql语句，在后面加入INTEGER NOT NULL DEFAULT 0 保证当它为null时赋予0，否则Room会直接抛异常；其他的报错基本能从报错信息理解出来。
       顺便晒出AppDatabase代码

@Database(entities = [HistoryEntity::class, CollectEntity::class], version = 4)
abstract class AppDatabase : RoomDatabase() {

    abstract fun historyDao(): HistoryDao
    abstract fun collectDao(): CollectionDao

    companion object {

        @Volatile
        private var INSTANCE: AppDatabase? = null


        fun getInstance(): AppDatabase =
            INSTANCE ?: synchronized(this) {
                INSTANCE ?: buildDatabase(App.instance).also { INSTANCE = it }
            }

        private fun buildDatabase(context: Context) =
            Room.databaseBuilder(
                context.applicationContext,
                AppDatabase::class.java, "exercise.db"
            ).addMigrations(MIGRATION_1_2, MIGRATION_2_3, MIGRATION_3_4)
                .build()

        private val MIGRATION_1_2: Migration = object : Migration(1, 2) {
            override fun migrate(database: SupportSQLiteDatabase) {
                database.execSQL("ALTER TABLE history ADD COLUMN update_time INTEGER NOT NULL DEFAULT 0")
            }
        }

        private val MIGRATION_2_3: Migration = object : Migration(2, 3) {
            override fun migrate(database: SupportSQLiteDatabase) {
                database.execSQL(
                    "CREATE TABLE collect (collect_id INTEGER NOT NULL DEFAULT 0,line_name_collect TEXT NOT NULL DEFAULT '', upper_or_down_collect TEXT NOT NULL DEFAULT '', PRIMARY KEY(collect_id))"
                )
            }
        }

        private val MIGRATION_3_4: Migration = object : Migration(3, 4) {
            override fun migrate(database: SupportSQLiteDatabase) {
                database.execSQL(
                    "CREATE TABLE collect_new_one (collect_id INTEGER NOT NULL DEFAULT 0,line_name_collect TEXT NOT NULL DEFAULT '', upper_or_down_collect TEXT NOT NULL DEFAULT '', PRIMARY KEY(collect_id))"
                )
                database.execSQL(
                    "INSERT INTO collect_new_one (collect_id , line_name_collect, upper_or_down_collect) SELECT collect_id, line_name_collect, upper_or_down_collect FROM collect"
                )
                database.execSQL("DROP TABLE collect")
                database.execSQL("ALTER TABLE collect_new_one RENAME TO collect")
            }
        }
    }
}

总结

       这是我自己练手项目中截取的一个记录，项目里使用到jetpack，后面有时间还会一一记录与分享，对于Room的简单使用上总体感觉还算方便快速，就是这个数据迁移会踩点坑，比较麻烦