Room database setup in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Room in Android development?

Room is a library that provides an easy way to work with SQLite databases by using simple annotations and abstracting boilerplate code.

beginner

Which annotation is used to define an entity (table) in Room?

The @Entity annotation is used to mark a Kotlin data class as a table in the Room database.

beginner

What is the role of the @Dao interface in Room?

The @Dao interface defines methods to access the database, such as queries, inserts, updates, and deletes.

intermediate

How do you create the Room database instance in your app?

You create the database instance using Room.databaseBuilder(context, YourDatabase::class.java, "database_name").build() inside a singleton or application class.

intermediate

Why should Room database operations be done off the main thread?

Because database operations can be slow, doing them on the main thread can freeze the UI. Room enforces this by throwing an error if you try to do heavy operations on the main thread.

Which annotation marks a data class as a table in Room?

A @Database

B @Dao

C @Table

D @Entity

The @Entity annotation is used to define a table in Room.

What does @Dao stand for in Room?

A Data Access Object

B Database Access Operation

C Data Annotation Object

D Direct Access Operation

@Dao stands for Data Access Object, which defines database interaction methods.

How do you create a Room database instance?

A new RoomDatabase()

B Room.databaseBuilder(context, DbClass::class.java, "name").build()

C Room.createDatabase()

D DatabaseBuilder.create()

Room.databaseBuilder() is the correct way to create a database instance.

Which of these is NOT a recommended practice with Room?

A Running queries on the main thread

B Using @Entity for tables

C Defining DAO interfaces

D Using LiveData with queries

Running queries on the main thread can freeze the UI and is not recommended.

What is the purpose of the @Database annotation?

A To create tables automatically

B To mark a class as a DAO

C To define the database holder and list entities

D To run queries

@Database annotation defines the database class and its entities.

Explain the main components needed to set up a Room database in an Android app.

Think about the classes and annotations you need to define tables, access data, and create the database.

Why is it important to avoid running Room database operations on the main thread? How can you handle this?

Consider what happens if the app waits too long for data and how Kotlin helps with background work.

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