Why Database migrations in Android Kotlin? - Purpose & Use Cases

The Big Idea

What if your app could upgrade its database without ever losing a single user's data?

The Scenario

Imagine you have an app with a database storing user info. You want to add a new feature that needs extra data fields. Without migrations, you must delete the old database and start fresh, losing all user data.

The Problem

Manually changing database structure is slow and risky. You might forget to copy data, cause app crashes, or confuse users with lost info. It's like remodeling a house by tearing down walls without a plan.

The Solution

Database migrations let you update your database step-by-step without losing data. They apply changes safely and automatically when the app updates, keeping user info intact and app stable.

Before vs After

✗ Before

db.execSQL("DROP TABLE users")
db.execSQL("CREATE TABLE users (id INTEGER PRIMARY KEY)")

✓ After

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

What It Enables

With migrations, your app can grow and improve its database without breaking or losing user data.

Real Life Example

A social app adds a new "last seen" timestamp to user profiles. Using migrations, it updates the database smoothly for millions of users without data loss or crashes.

Key Takeaways

Manual database changes risk data loss and app crashes.

Migrations update database safely and automatically.

Migrations keep user data intact during app upgrades.

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