Creating Many-to-Many Relationships in Room with Jetpack Compose

Jetpack Compose, Google's modern toolkit for building native Android UIs, pairs seamlessly with Room, the database library that simplifies data persistence on Android. While Room natively supports one-to-one and one-to-many relationships, implementing many-to-many relationships requires a bit more effort. This blog post explores how to effectively create and manage many-to-many relationships in Room and leverage them in Jetpack Compose applications.

Understanding Many-to-Many Relationships in Room

A many-to-many relationship allows multiple entities of one type to be associated with multiple entities of another type. For example, consider a scenario where we have Student and Course entities:

• A student can enroll in multiple courses.

• A course can have multiple students.

To model this in Room, we need an intermediate (or junction) table that establishes the relationship between these two entities.

Setting Up the Data Model

1. Defining the Entities

We’ll start by creating the Student and Course entities.

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

@Entity
data class Course(
    @PrimaryKey(autoGenerate = true) val courseId: Long = 0,
    val courseName: String
)

2. Creating the Junction Table

The junction table establishes the many-to-many relationship between Student and Course.

@Entity(
    primaryKeys = ["studentId", "courseId"],
    foreignKeys = [
        ForeignKey(
            entity = Student::class,
            parentColumns = ["studentId"],
            childColumns = ["studentId"],
            onDelete = ForeignKey.CASCADE
        ),
        ForeignKey(
            entity = Course::class,
            parentColumns = ["courseId"],
            childColumns = ["courseId"],
            onDelete = ForeignKey.CASCADE
        )
    ]
)
data class StudentCourseCrossRef(
    val studentId: Long,
    val courseId: Long
)

3. Establishing Relationships with Data Classes

To query and work with the relationships, we’ll use data classes that combine the entities.

Students with Their Courses

data class StudentWithCourses(
    @Embedded val student: Student,
    @Relation(
        parentColumn = "studentId",
        entityColumn = "courseId",
        associateBy = Junction(StudentCourseCrossRef::class)
    )
    val courses: List<Course>
)

Courses with Their Students

data class CourseWithStudents(
    @Embedded val course: Course,
    @Relation(
        parentColumn = "courseId",
        entityColumn = "studentId",
        associateBy = Junction(StudentCourseCrossRef::class)
    )
    val students: List<Student>
)

Creating the DAO

The DAO provides the interface for interacting with the database. Here are the queries we’ll use:

@Dao
interface SchoolDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertStudent(student: Student)

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertCourse(course: Course)

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insertStudentCourseCrossRef(crossRef: StudentCourseCrossRef)

    @Transaction
    @Query("SELECT * FROM Student WHERE studentId = :studentId")
    suspend fun getStudentWithCourses(studentId: Long): List<StudentWithCourses>

    @Transaction
    @Query("SELECT * FROM Course WHERE courseId = :courseId")
    suspend fun getCourseWithStudents(courseId: Long): List<CourseWithStudents>
}

Integrating Room with Jetpack Compose

Now that we’ve set up the database layer, let’s move to the UI layer. Jetpack Compose makes it intuitive to display and interact with data.

Setting Up ViewModel

Use a ViewModel to fetch data and expose it to the UI.

class SchoolViewModel(application: Application) : AndroidViewModel(application) {
    private val schoolDao = Room.databaseBuilder(
        application,
        SchoolDatabase::class.java,
        "school_db"
    ).build().schoolDao()

    private val _studentsWithCourses = mutableStateOf<List<StudentWithCourses>>(emptyList())
    val studentsWithCourses: State<List<StudentWithCourses>> = _studentsWithCourses

    fun fetchStudentWithCourses(studentId: Long) {
        viewModelScope.launch {
            _studentsWithCourses.value = schoolDao.getStudentWithCourses(studentId)
        }
    }
}

Displaying Data in Compose

Here’s how to display the list of students with their courses:

@Composable
fun StudentCoursesScreen(viewModel: SchoolViewModel, studentId: Long) {
    val studentsWithCourses by viewModel.studentsWithCourses.observeAsState(emptyList())

    LaunchedEffect(studentId) {
        viewModel.fetchStudentWithCourses(studentId)
    }

    LazyColumn {
        items(studentsWithCourses) { studentWithCourses ->
            Text("Student: ${studentWithCourses.student.name}")
            studentWithCourses.courses.forEach { course ->
                Text(" - Course: ${course.courseName}")
            }
        }
    }
}

Best Practices for Managing Many-to-Many Relationships

1. Normalize Your Database: Ensure that you’re using junction tables appropriately to avoid data duplication.

2. Leverage Transactions: Always wrap insertions and updates in transactions to maintain consistency.

3. Optimize Queries: Use appropriate indexes on the junction table to enhance query performance.

4. Avoid Over-fetching: Fetch only the data you need for the UI to avoid performance bottlenecks.

Advanced Use Cases

1. Dynamic Filtering: Add search and filter capabilities by creating custom queries in your DAO.

2. Data Synchronization: Use tools like WorkManager to synchronize many-to-many relationships with a remote server.

3. Custom UI Components: Create reusable Compose components for displaying nested data structures like many-to-many relationships.

Conclusion

Managing many-to-many relationships in Room can seem complex, but with a structured approach, it becomes manageable and efficient. Jetpack Compose further simplifies the process of displaying relational data, making it a perfect match for Room. By combining these tools, you can build powerful, data-driven Android applications with ease.

Start implementing these concepts in your next project to take full advantage of the flexibility and power of Jetpack Compose and Room.