Android Room 数据库迁移没有正确处理

Question

In my application, I am using a Room database. Here to support the offline feature, I need to use the pre-populated database.

So, what I need is, I have created one Room database with values, and put that database in the assets folder of the project and importing the database to my local.

Now everything works fine.

Now, for some reason, I am adding one more column to my local database ( not the external which is in assets folder ), but it gives me the following error.

java.lang.IllegalStateException: Migration didn't properly handle

I have also added the migration.

Following is my code for the database.

@Database(
entities = arrayOf(ModelCategories::class, ModelApps::class),
version = 4,
exportSchema = false)

abstract class LauncherDatabase : RoomDatabase() {

abstract fun categoriesDAO(): DAOCategories
abstract fun appsDAO(): DAOApps

companion object {
    // Singleton prevents multiple instances of database opening at the
    // same time.
    @Volatile
    private var INSTANCE: LauncherDatabase? = null

    /*
        When there is change in Databse structure new migration should be there
     */
    @JvmField
    val MIGRATION_3_2: Migration = object : Migration(3, 4) {
        override fun migrate(database: SupportSQLiteDatabase) {
        }
    }

    fun getDatabase(context: Context): LauncherDatabase {
        val tempInstance = INSTANCE
        if (tempInstance != null) {
            return tempInstance
        }
        synchronized(this) {
            val instance = Room.databaseBuilder(
                context.applicationContext,
                LauncherDatabase::class.java,
                "launcher_database"
            ).createFromAsset("databases/launcher_database.db")
                .addMigrations(LauncherDatabase.MIGRATION_3_2)
                .build()
            INSTANCE = instance
            return instance
        }
    }
}}

Note:- The database version of the external assets database is 3. So, after adding one column to Model class, I am increasing the current database version to 4.

Thanks in advance.

Answer 1

inside override fun migrate you have to write

database.execSQL("ALTER TABLE your_table_name ADD COLUMN column_name type");

where type will be Integer in case of int, TEXT in case of String or custom data type etc. If you can tell your column data type then I can add actual code.