Database migrations in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens after a database migration?

You have an Android app using Room database. You add a new column to a table and write a migration from version 1 to 2. What will the app do after updating if the migration is correct?

A The app crashes because the schema version is higher than before.

B The app deletes all existing data and creates a new database.

C The app updates the database schema without losing existing data.

D The app ignores the migration and uses the old schema.

💡 Hint

Think about what a migration is supposed to do to keep data safe.

A correct migration updates the database schema to the new version while preserving existing data. It applies changes like adding columns or tables without deleting data.

📝 Syntax

intermediate

Identify the correct Room migration code snippet

Which Kotlin code snippet correctly defines a migration from version 1 to 2 that adds a new column 'age' to the 'User' table with a default value 0?

Android Kotlin

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

A database.execSQL("DROP TABLE User")

B database.execSQL("ALTER TABLE User ADD COLUMN age INTEGER NOT NULL DEFAULT 0")

C database.execSQL("CREATE TABLE User(age INTEGER)")

D database.execSQL("ALTER TABLE User ADD COLUMN age TEXT")

💡 Hint

Adding a non-null column requires a default value in SQLite.

Option B correctly adds a new non-null INTEGER column with a default value, which is required to avoid errors during migration.

❓ lifecycle

advanced

When is the migration code executed in an Android app?

At what point during the app lifecycle does Room execute the migration code when upgrading the database version?

A When the app is uninstalled and reinstalled.

B Immediately when the app starts, before any UI loads.

C Only when the user triggers a manual database refresh.

D When the database is first opened or accessed after the app update.

💡 Hint

Think about when the app needs to access the database after an update.

Room runs migrations when the database is opened for the first time after an update, ensuring the schema is current before queries run.

🔧 Debug

advanced

Why does this migration cause a crash?

You wrote this migration code: val MIGRATION_2_3 = object : Migration(2, 3) { override fun migrate(database: SupportSQLiteDatabase) { database.execSQL("ALTER TABLE User ADD COLUMN email TEXT NOT NULL") } } The app crashes on upgrade. Why?

A Adding a NOT NULL column without a default value causes a constraint error.

B The SQL syntax for ALTER TABLE is incorrect.

C The migration version numbers are reversed.

D The User table does not exist in version 2.

💡 Hint

SQLite requires a default value when adding NOT NULL columns.

SQLite cannot add a NOT NULL column without a default value because existing rows would violate the constraint, causing a crash.

🧠 Conceptual

expert

What is the best practice for handling complex schema changes in Room migrations?

You need to rename a column and change its type in a Room database migration. SQLite does not support renaming columns directly. What is the recommended approach?

A Create a new table with the desired schema, copy data from the old table, drop the old table, then rename the new table.

B Delete the database and recreate it with the new schema.

C Use ALTER TABLE to rename the column and change its type directly.

D Ignore the change and keep the old schema.

💡 Hint

Think about how to preserve data when SQLite lacks direct support for renaming columns.

Since SQLite does not support renaming columns or changing types directly, the best practice is to create a new table with the new schema, copy data over, drop the old table, and rename the new one.