Database migrations in Android Kotlin

Introduction

Database migrations help you change your app's database safely when you update your app. They keep your data safe and your app working well.

You add a new column to a table in your app's database.

You rename or remove a table or column in your database.

You change the data type of a column in your database.

You want to fix a mistake in your database structure after releasing your app.

You add new tables to support new features in your app.

Syntax

Android Kotlin

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

This example shows a migration from version 1 to 2.

The migrate function runs SQL commands to update the database.

Examples

This migration adds a new table called orders in version 3.

Android Kotlin

val MIGRATION_2_3 = object : Migration(2, 3) {
  override fun migrate(database: SupportSQLiteDatabase) {
    database.execSQL("CREATE TABLE orders (id INTEGER PRIMARY KEY NOT NULL, userId INTEGER, amount REAL)")
  }
}

This migration removes a temporary table temp_data in version 4.

Android Kotlin

val MIGRATION_3_4 = object : Migration(3, 4) {
  override fun migrate(database: SupportSQLiteDatabase) {
    database.execSQL("DROP TABLE IF EXISTS temp_data")
  }
}

Sample App

This example shows a Room database with a migration from version 1 to 2. It adds a new column age to the user table. The migration is added when building the database.

Android Kotlin

import androidx.room.Room
import androidx.room.RoomDatabase
import androidx.room.Database
import androidx.room.migration.Migration
import androidx.sqlite.db.SupportSQLiteDatabase

@Database(entities = [User::class], version = 2)
abstract class AppDatabase : RoomDatabase() {
  abstract fun userDao(): UserDao
}

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

// Usage example
val db = Room.databaseBuilder(context, AppDatabase::class.java, "app-db")
  .addMigrations(MIGRATION_1_2)
  .build()

Important Notes

Always test migrations on real data to avoid data loss.

Each migration handles one step between two versions.

If you skip versions, you must provide migrations for each step or use fallback strategies.

Summary

Database migrations update your app's database without losing data.

They run SQL commands to change tables or columns safely.

Use migrations whenever you change your database schema.

Practice

1.

What is the main purpose of a database migration in Android apps?

easy

A. To update the database schema without losing existing data

B. To delete all data and recreate the database from scratch

C. To backup the database to external storage

D. To optimize database queries for faster performance

Solution

1. Step 1: Understand database migration purpose

   Database migrations are used to change the structure of the database safely.

2. Step 2: Identify data preservation goal

   Migrations update tables or columns without deleting existing user data.

3. Final Answer:

   To update the database schema without losing existing data -> Option A

4. Quick Check:

   Migration = Safe schema update [OK]

Hint: Migrations keep data safe while changing schema [OK]

Common Mistakes:

• Thinking migrations delete all data
• Confusing migrations with backups
• Believing migrations only optimize queries

2.

Which of the following is the correct way to define a migration from version 1 to 2 in Room database?

val MIGRATION_1_2 = object : Migration(1, 2) {
    override fun migrate(database: SupportSQLiteDatabase) {
        // ?
    }
}

easy

A. database.execSQL("ALTER TABLE users ADD COLUMN age INTEGER DEFAULT 0")

B. database.update("users", "age", 0)

C. database.insert("users", ContentValues())

D. database.delete("users")

Solution

1. Step 1: Understand Migration class usage

   Migration requires SQL commands inside migrate() to change schema.

2. Step 2: Identify correct SQL command

   ALTER TABLE adds a new column safely; execSQL runs raw SQL.

3. Final Answer:

   database.execSQL("ALTER TABLE users ADD COLUMN age INTEGER DEFAULT 0") -> Option A

4. Quick Check:

   Migration uses execSQL with ALTER TABLE [OK]

Hint: Use execSQL with ALTER TABLE to add columns [OK]

Common Mistakes:

• Using update() instead of execSQL
• Trying to insert or delete rows in migration
• Not providing default value for new column

3.

Given this migration code, what will be the result after migrating from version 1 to 2?

val MIGRATION_1_2 = object : Migration(1, 2) {
    override fun migrate(database: SupportSQLiteDatabase) {
        database.execSQL("CREATE TABLE IF NOT EXISTS books (id INTEGER PRIMARY KEY, title TEXT)")
    }
}

Assuming version 1 had no books table.

medium

A. No changes happen because CREATE TABLE is ignored

B. Migration fails because table 'books' already exists

C. A new table 'books' is created with columns 'id' and 'title'

D. Existing tables are deleted and only 'books' remains

Solution

1. Step 1: Analyze migration SQL command

   CREATE TABLE IF NOT EXISTS creates the table only if it doesn't exist.

2. Step 2: Check initial database state

   Version 1 has no 'books' table, so it will be created.

3. Final Answer:

   A new table 'books' is created with columns 'id' and 'title' -> Option C

4. Quick Check:

   CREATE TABLE IF NOT EXISTS creates missing table [OK]

Hint: IF NOT EXISTS avoids errors if table exists [OK]

Common Mistakes:

• Assuming migration fails if table exists
• Thinking CREATE TABLE is ignored
• Believing existing tables get deleted

4.

What is wrong with this migration code from version 2 to 3?

val MIGRATION_2_3 = object : Migration(2, 3) {
    override fun migrate(database: SupportSQLiteDatabase) {
        database.execSQL("ALTER TABLE users ADD COLUMN email TEXT")
        database.execSQL("ALTER TABLE users ADD COLUMN email TEXT")
    }
}

medium

A. The migration does not update the database version

B. The same column 'email' is added twice causing an error

C. The SQL syntax for ALTER TABLE is incorrect

D. The migration should delete the 'users' table first

Solution

1. Step 1: Review SQL commands in migration

   The code tries to add the 'email' column twice to the same table.

2. Step 2: Understand SQLite behavior

   Adding the same column twice causes a runtime error during migration.

3. Final Answer:

   The same column 'email' is added twice causing an error -> Option B

4. Quick Check:

   Duplicate column addition causes migration failure [OK]

Hint: Avoid adding same column twice in one migration [OK]

Common Mistakes:

• Ignoring duplicate column errors
• Thinking migration version is updated automatically
• Believing ALTER TABLE syntax is wrong here

5.

You want to rename a column username to user_name in your Room database from version 3 to 4. SQLite does not support renaming columns directly. Which migration strategy is correct?

hard

A. Add a new column and leave the old column as is

B. Use ALTER TABLE to rename the column directly

C. Delete the old table and create a new one with the new column name

D. Create a new table with the new column name, copy data, drop old table, rename new table

Solution

1. Step 1: Understand SQLite column rename limitation

   SQLite does not support direct column renaming with ALTER TABLE.

2. Step 2: Apply recommended migration pattern

   Create a new table with desired schema, copy data, drop old table, rename new table.

3. Final Answer:

   Create a new table with the new column name, copy data, drop old table, rename new table -> Option D

4. Quick Check:

   Rename column = new table + copy data [OK]

Hint: Rename columns by new table + copy data [OK]

Common Mistakes:

• Trying to rename column with ALTER TABLE
• Deleting old table without copying data
• Adding new column but not removing old one