Room database setup in Android Kotlin

Introduction

Room helps you save and manage app data easily on your phone. It makes working with databases simple and safe.

You want to save user notes or settings locally on the device.

You need to keep a list of items like tasks or contacts that stay after the app closes.

You want to store data offline and sync later when internet is available.

Syntax

Android Kotlin

1. Add Room dependencies in build.gradle
2. Create an @Entity data class for your table
3. Create a @Dao interface with database operations
4. Create an abstract @Database class extending RoomDatabase
5. Build the database instance using Room.databaseBuilder()

Use @Entity to define a table in the database.

Use @Dao to define methods for inserting, querying, updating, and deleting.

Examples

Defines a User table with id as the primary key and a name column.

Android Kotlin

@Entity
data class User(
  @PrimaryKey val id: Int,
  val name: String
)

Defines methods to add a user and get all users from the User table.

Android Kotlin

@Dao
interface UserDao {
  @Insert
  suspend fun insert(user: User)

  @Query("SELECT * FROM User")
  suspend fun getAll(): List<User>
}

Creates the Room database class linking the User entity and UserDao.

Android Kotlin

@Database(entities = [User::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
  abstract fun userDao(): UserDao
}

Sample App

This example shows how to set up Room with a User table, insert a user, and fetch all users.

Android Kotlin

import android.content.Context
import androidx.room.*

@Entity
data class User(
  @PrimaryKey val id: Int,
  val name: String
)

@Dao
interface UserDao {
  @Insert
  suspend fun insert(user: User)

  @Query("SELECT * FROM User")
  suspend fun getAll(): List<User>
}

@Database(entities = [User::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
  abstract fun userDao(): UserDao
}

// Usage example in a coroutine scope
suspend fun demoRoom(context: Context) {
  val db = Room.databaseBuilder(
    context.applicationContext,
    AppDatabase::class.java, "app-db"
  ).build()

  val userDao = db.userDao()
  userDao.insert(User(1, "Alice"))
  val users = userDao.getAll()
  println(users.joinToString())
}

Important Notes

Always run database operations off the main thread to avoid freezing the app UI.

Use suspend functions or other threading methods to handle database calls.

Room generates code for you, so you don't write raw SQL queries manually.

Summary

Room simplifies local data storage in Android apps.

Define entities, DAOs, and the database class to set up Room.

Use Room.databaseBuilder() to create the database instance.

Practice

1. What is the main purpose of the @Entity annotation in Room database setup?

easy

A. To define a table in the database

B. To create a database instance

C. To write SQL queries

D. To start a database transaction

Solution

1. Step 1: Understand the role of @Entity

   The @Entity annotation marks a Kotlin data class as a table in the Room database.

2. Step 2: Differentiate from other annotations

   Other annotations like @Dao define data access objects, not tables.

3. Final Answer:

   To define a table in the database -> Option A

4. Quick Check:

   @Entity = Table definition [OK]

Hint: Remember: @Entity means table in Room [OK]

Common Mistakes:

• Confusing @Entity with @Dao
• Thinking @Entity creates the database
• Mixing @Entity with SQL query annotations

2. Which of the following is the correct way to declare a Room database class in Kotlin?

easy

A. @Dao(entities = [User::class], version = 1)\ninterface AppDatabase {\n fun userDao(): UserDao\n}

B. @Database(entities = [User::class], version = 1)\nabstract class AppDatabase : RoomDatabase() {\n abstract fun userDao(): UserDao\n}

C. @Entity(entities = [User::class], version = 1)\nclass AppDatabase : RoomDatabase() {\n fun userDao(): UserDao\n}

D. @Database(entities = User, version = 1)\nclass AppDatabase() {\n fun userDao(): UserDao\n}

Solution

1. Step 1: Identify correct annotation and class type

   The Room database class must be annotated with @Database and extend RoomDatabase.

2. Step 2: Check syntax for entities and abstract DAO method

   The entities list must be an array of classes, and DAO methods must be abstract.

3. Final Answer:

   @Database(entities = [User::class], version = 1)\nabstract class AppDatabase : RoomDatabase() {\n abstract fun userDao(): UserDao\n} -> Option B

4. Quick Check:

   @Database + abstract class + extends RoomDatabase = correct [OK]

Hint: Database class is abstract and annotated @Database [OK]

Common Mistakes:

• Using @Entity instead of @Database
• Not making the class abstract
• Forgetting to extend RoomDatabase

3. Given the following code snippet, what will be the result of calling db.userDao().getAllUsers()?

@Dao
interface UserDao {
  @Query("SELECT * FROM user")
  fun getAllUsers(): List<User>
}

@Entity
data class User(
  @PrimaryKey val id: Int,
  val name: String
)

// Assume database is properly created and contains 3 users

medium

A. A runtime error because of missing suspend keyword

B. An empty list

C. A list of 3 User objects from the database

D. A compilation error due to missing @Insert method

Solution

1. Step 1: Understand the DAO query

   The @Query("SELECT * FROM user") fetches all rows from the user table.

2. Step 2: Check the database state

   The question states the database contains 3 users, so the query returns 3 User objects.

3. Final Answer:

   A list of 3 User objects from the database -> Option C

4. Quick Check:

   Query returns all users = 3 users [OK]

Hint: Query returns all rows; count matches stored data [OK]

Common Mistakes:

• Assuming empty list without data check
• Confusing suspend functions with normal DAO methods
• Expecting errors without code context

4. Identify the error in this Room database builder code snippet:

val db = Room.databaseBuilder(
  context,
  AppDatabase::class.java,
  "app_db"
).build()

medium

A. Missing fallbackToDestructiveMigration() causing crash on schema change

B. Incorrect database name format, should be a URI

C. AppDatabase must be an interface, not a class

D. databaseBuilder requires a coroutine scope parameter

Solution

1. Step 1: Review databaseBuilder usage

   The code correctly builds the database but lacks migration strategy.

2. Step 2: Understand migration impact

   Without fallbackToDestructiveMigration(), schema changes cause runtime crashes.

3. Final Answer:

   Missing fallbackToDestructiveMigration() causing crash on schema change -> Option A

4. Quick Check:

   Missing migration fallback = crash risk [OK]

Hint: Add fallbackToDestructiveMigration() to avoid crashes [OK]

Common Mistakes:

• Thinking database name must be URI
• Confusing class vs interface for database
• Expecting coroutine scope in builder

5. You want to add a new column age to your existing User entity without losing existing data. Which approach correctly updates the Room database?

hard

A. Change the database name to force a new database creation

B. Just add the new field in the User class and keep the same database version

C. Delete the app data and reinstall to reset the database

D. Increase database version and provide a migration object with SQL to add the new column

Solution

1. Step 1: Understand schema changes in Room

   Adding a new column changes the database schema, so version must increase.

2. Step 2: Provide migration to preserve data

   Migration object with SQL ALTER TABLE adds the column without losing data.

3. Final Answer:

   Increase database version and provide a migration object with SQL to add the new column -> Option D

4. Quick Check:

   Schema change needs version bump + migration [OK]

Hint: Always migrate with version bump to keep data safe [OK]

Common Mistakes:

• Changing entity without version update
• Deleting app data instead of migrating
• Changing database name to avoid migration