Setting Up One-to-Many Relationships in Room with Jetpack Compose

Jetpack Compose is revolutionizing Android UI development with its declarative approach, and when combined with Room, it creates a robust stack for handling local data persistence in modern Android apps. One-to-many relationships are a common use case, enabling efficient representation and management of data, such as customers and their orders or authors and their books. This blog post dives deep into setting up one-to-many relationships in Room and seamlessly integrating them with Jetpack Compose.

What is a One-to-Many Relationship in Room?

In the context of databases, a one-to-many relationship means that a single record in one table (the parent) is associated with multiple records in another table (the child). For example, an Author can write multiple Books. Room, as part of Android Jetpack, provides an abstraction layer over SQLite, simplifying the management of such relationships.

Key Concepts in Room Relationships

Before diving into implementation, let’s clarify some essential concepts:

  • Entity: Represents a table in the database.

  • Foreign Key: Links one table to another, establishing the relationship.

  • Embedded Relationships: Room supports relationships using the @Relation annotation.

  • DAO (Data Access Object): Defines methods for interacting with the database.

Step 1: Define the Database Entities

Let’s create two entities: Author and Book.

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

@Entity
data class Author(
    @PrimaryKey(autoGenerate = true) val authorId: Long = 0,
    val name: String
)

@Entity(
    foreignKeys = [
        ForeignKey(
            entity = Author::class,
            parentColumns = ["authorId"],
            childColumns = ["authorId"],
            onDelete = ForeignKey.CASCADE
        )
    ]
)
data class Book(
    @PrimaryKey(autoGenerate = true) val bookId: Long = 0,
    val title: String,
    val authorId: Long
)

  • The Author entity includes a unique authorId.

  • The Book entity includes a foreign key authorId, linking it to the Author table.

  • The onDelete = ForeignKey.CASCADE ensures that deleting an Author also removes associated Books.

Step 2: Define the Relationship

Use Room’s @Relation annotation to define the one-to-many relationship.

import androidx.room.Embedded
import androidx.room.Relation

data class AuthorWithBooks(
    @Embedded val author: Author,
    @Relation(
        parentColumn = "authorId",
        entityColumn = "authorId"
    )
    val books: List<Book>
)

  • The @Embedded annotation includes the Author fields.

  • The @Relation annotation fetches the associated Book records based on the authorId.

Step 3: Create the DAO

Define the methods to retrieve and insert data.

import androidx.room.Dao
import androidx.room.Insert
import androidx.room.Query
import androidx.room.Transaction

@Dao
interface AuthorBookDao {
    @Insert
    suspend fun insertAuthor(author: Author): Long

    @Insert
    suspend fun insertBook(book: Book)

    @Transaction
    @Query("SELECT * FROM Author WHERE authorId = :authorId")
    suspend fun getAuthorWithBooks(authorId: Long): AuthorWithBooks
}

  • The @Insert methods handle data insertion.

  • The @Transaction ensures atomicity when fetching related data.

Step 4: Set Up the Database

Integrate the entities and DAO into the Room database.

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

@Database(entities = [Author::class, Book::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
    abstract fun authorBookDao(): AuthorBookDao
}

Step 5: Integrate with Jetpack Compose

With the Room setup complete, it’s time to leverage Jetpack Compose to display the data.

Displaying Data in a LazyColumn

Use LazyColumn to display authors and their books.

import androidx.compose.foundation.layout.*
import androidx.compose.material3.*
import androidx.compose.runtime.*
import androidx.compose.ui.Modifier
import androidx.lifecycle.viewmodel.compose.viewModel
import kotlinx.coroutines.launch

@Composable
fun AuthorBooksScreen(viewModel: AuthorBookViewModel = viewModel()) {
    val authorWithBooks by viewModel.authorWithBooks.collectAsState(initial = null)

    Column(modifier = Modifier.fillMaxSize()) {
        authorWithBooks?.let { authorBooks ->
            Text(text = authorBooks.author.name, style = MaterialTheme.typography.titleLarge)

            LazyColumn(modifier = Modifier.fillMaxSize()) {
                items(authorBooks.books) { book ->
                    Text(text = book.title, style = MaterialTheme.typography.bodyLarge)
                }
            }
        }
    }
}
ViewModel for Data Handling

Use a ViewModel to fetch data from the Room database.

import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.StateFlow
import kotlinx.coroutines.launch

class AuthorBookViewModel(private val dao: AuthorBookDao) : ViewModel() {
    private val _authorWithBooks = MutableStateFlow<AuthorWithBooks?>(null)
    val authorWithBooks: StateFlow<AuthorWithBooks?> get() = _authorWithBooks

    fun loadAuthorWithBooks(authorId: Long) {
        viewModelScope.launch {
            _authorWithBooks.value = dao.getAuthorWithBooks(authorId)
        }
    }
}

Best Practices for One-to-Many Relationships

  • Use Cascading Deletes: Define onDelete = ForeignKey.CASCADE to maintain referential integrity.

  • Optimize Queries: Use @Transaction for related data retrieval to ensure atomicity.

  • Leverage Paging: For large datasets, integrate Room with Paging 3 for efficient data loading.

  • UI State Management: Use StateFlow or LiveData for reactive UI updates in Jetpack Compose.

Conclusion

Setting up one-to-many relationships in Room and integrating them with Jetpack Compose can significantly enhance your app’s data handling and user experience. By following these steps, you can create a maintainable, efficient architecture that scales with your app’s complexity. Whether you’re building a simple list or a feature-rich database app, Room and Jetpack Compose provide the tools you need to succeed.

Jetpack Compose and Room are essential parts of modern Android development. Mastering their integration will give you a significant edge in creating dynamic and efficient applications.