Saving Data on Android

RENAME TABLE
, 
RENAME COLUMN
 
and 
ADD COLUMN
.
If you want to perform complex schema changes such as changing the type affinity 
of a column, you'll need to use more than one query. But don't worry, the following 
steps summarize the process:
1. Create a new temporary table with the new schema.
2. Copy the data from the original table to the temporary table.
3. Drop the original table.
4. Rename the temporary table with the same name as the original table.
To illustrate this process, imagine you want to modify the type affinity of the 
difficulty column to TEXT instead of INTEGER so that you can store the operating 
system that the question refers to.
To do this, open 
Question.kt
  and modify the category property:
@Entity(tableName = "question", indices = 
[Index("question_id")
]) 
data
 
class
 
Question
( 
  
@PrimaryKey(autoGenerate = true)
 
  
@ColumnInfo(name = "question_id")
 
  
var
 questionId: 
Int
, 
  
val
 text: String, 
  
val
 difficulty: String = 
"0"
, 
//Only this line changes  
 
  
val
 category: String = 
"android"
 
)
Now, create a new class under the migrations package and name it 
Migration3To4
. 
Modify everything like this:
class
 
Migration3To4
 : 
Migration
(
3
, 
4
) { 
  
override
 
fun
 
migrate
(database: SupportSQLiteDatabase)
 { 
    database.execSQL( 
      
"CREATE TABLE question_new (question_id INTEGER NOT NULL, 
"
 + 
          
"text TEXT NOT NULL, "
 + 
          
"difficulty TEXT NOT NULL, "
 + 
          
"category TEXT NOT NULL, "
 + 
          
"PRIMARY KEY (question_id))"
 
    ) 
//1
 
 
    database.execSQL(
"CREATE INDEX 
index_question_new_question_id ON question_new(question_id)"
) 
//
Saving Data on Android
Chapter 10: Migrations with Room
raywenderlich.com
182

---

2
 
 
    database.execSQL( 
      
"INSERT INTO question_new (question_id, text, difficulty, 
category) "
 + 
          
"SELECT question_id, text, difficulty, category FROM 
question"
 
    )
//3
 
 
    database.execSQL(
"DROP TABLE question"
) 
//4
 
 
    database.execSQL(
"ALTER TABLE question_new RENAME TO 
question"
) 
//5
 
 
  } 
}
Briefly:
1. Creates a new temporary table with the new schema called