Saving Data on Android

question_new
.
2. Adds an index to the 
question_id
 column of your 
question_new
 table.
3. Retrieves all the data from your original 
question
 table using a 
SELECT
 
statement and adds it to your 
question_new
 table using an 
INSERT INTO
 
statement.
4. Drops the original question table using a 
DROP TABLE
 statement.
5. Renames your 
question_new
 table to 
question
 using an 
ALTER TABLE
 with a 
RENAME TO
 statement.
Open the 
QuizDatabase
 class and modify it like this:
@Database(entities = [(Question::class)
, (Answer::
class
)], 
version = 4)//Changes the db version
 
abstract
 
class
 
QuizDatabase
 : 
RoomDatabase
() { 
  
companion
 
object
{ 
    
val
 MIGRATION_1_TO_2 = Migration1To2() 
    
val
 MIGRATION_2_TO_3 = Migration2To3() 
    
val
 MIGRATION_3_TO_4 = Migration3To4() 
//Adds a reference to 
your new migration
 
  } 
  
abstract
 
fun
 
questionsDao
()
: QuestionDao 
}
Just like before, you've changed the database version to 4 and created a reference to 
your new migration inside the companion object.
Saving Data on Android
Chapter 10: Migrations with Room
raywenderlich.com
183

---

Finally, add your new migration to your database by opening the 
QuizApplication.kt
 file and changing your database builder inside 
onCreate()
:
database = Room.databaseBuilder(
this
, QuizDatabase::
class
.
java
, 
"question_database")
 
    .addMigrations(QuizDatabase.MIGRATION_1_TO_2, 
QuizDatabase.MIGRATION_2_TO_3, QuizDatabase.MIGRATION_3_TO_4) 
    .build()
Build and run your app, then press 
START
:
Cool!
You might have noticed that you now have three different migrations for four 
different versions of your database. If one of your users had the first version of your 
database installed and wanted to update the app to the latest version, Room would 
execute each migration one by one. Since four is still a relatively low number, the 
process should be quick, but imagine if you had 50 versions of your database! It 
would be much better to have a shortcut right?
Well, Room allows you to define a migration path that starts from and goes to any 
version of your database. To illustrate this concept, define a migration that goes 
from database version 1 to 4.
Saving Data on Android
Chapter 10: Migrations with Room
raywenderlich.com
184

---

Create a new class under the migrations package and name it 
Migration1To4
. 
Replace everything inside with the following:
class
 
Migration1To4
 : 
Migration
(
1
, 
4
) { 
  
override
 
fun
 
migrate
(database: SupportSQLiteDatabase)
 { 
    database.execSQL(
"ALTER TABLE question ADD COLUMN difficulty 
TEXT NOT NULL DEFAULT '0'"
) 
    database.execSQL(
"ALTER TABLE question ADD COLUMN category 
TEXT NOT NULL DEFAULT 'android'"
) 
  } 
}
The above simply executes two 
ALTER TABLE
 statements to add difficulty and 
category columns. Since these columns didn't exist in the version 1 database, we 
don't have to worry about the more complex SQL for the migration in the prior step.
Open the 
QuizDatabase
 class and add the following line to your companion object 
to create a reference to your new migration:
val
 MIGRATION_1_TO_4 = Migration1To4()
Now go to the