Room database setup in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a Room entity with a primary key.

Android Kotlin

import androidx.room.Entity
import androidx.room.PrimaryKey

@Entity
 data class User(
    @PrimaryKey val [1]: Int,
    val name: String
)

Drag options to blanks, or click blank then click option'

Aid

BuserId

Cuid

Dkey

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field name that is not declared in the data class.

Forgetting to annotate the primary key.

The primary key field is commonly named id to uniquely identify each entity.

2fill in blank

medium

Complete the code to create a DAO interface with a query to get all users.

Android Kotlin

import androidx.room.Dao
import androidx.room.Query

@Dao
interface UserDao {
    @Query("SELECT * FROM user")
    fun [1](): List<User>
}

Drag options to blanks, or click blank then click option'

AgetAllUsers

BfetchUsers

CloadAll

DretrieveUsers

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names that do not match the query purpose.

Omitting the return type.

The method name getAllUsers clearly describes the action of fetching all user records.

3fill in blank

hard

Fix the error in the Room database abstract class declaration.

Android Kotlin

import androidx.room.Database
import androidx.room.RoomDatabase

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

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ADatabaseBuilder

BDatabase

CRoomBuilder

DRoomDatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect superclass names.

Forgetting to make the class abstract.

The Room database class must extend RoomDatabase to work properly.

4fill in blank

hard

Fill both blanks to build the Room database instance in a singleton pattern.

Android Kotlin

import android.content.Context
import androidx.room.Room

object DatabaseBuilder {
    private var INSTANCE: AppDatabase? = null

    fun getInstance(context: Context): AppDatabase {
        if (INSTANCE == null) {
            INSTANCE = Room.[1](context.applicationContext, AppDatabase::class.java, "app_db").[2]()
        }
        return INSTANCE!!
    }
}

Drag options to blanks, or click blank then click option'

AdatabaseBuilder

Bbuild

Ccreate

Dinitialize

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names for building the database.

Not calling build() to finalize the database instance.

The method databaseBuilder starts building the database, and build() finalizes the creation.

5fill in blank

hard

Fill all three blanks to define an entity with an auto-generated primary key and a non-null column.

Android Kotlin

import androidx.room.Entity
import androidx.room.PrimaryKey
import androidx.room.ColumnInfo

@Entity
 data class Product(
    @PrimaryKey(autoGenerate = [1]) val id: Int = 0,
    @ColumnInfo(name = "product_name") val [2]: String,
    val price: Double = [3]
)

Drag options to blanks, or click blank then click option'

Atrue

Bfalse

Cname

D0.0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting autoGenerate to false or missing it.

Using wrong field names.

Not providing a default value for price.

Setting autoGenerate = true enables automatic ID generation. The product name field is named name. The price defaults to 0.0.

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