Is there a way to make relation to Set<String> in Room? - android-room

Question

Assume tables:
CREATE TABLE users (id INT, name VARCHAR(255))
CREATE TABLE user_roles (user_id INT REFERENCES users(id), role_name VARCHAR(255))
Entity in Room:
#Entity(tableName = "users")
data class User(
#PrimaryKey
val id: Int,
val roles: Set<String> // want it from role_name column
)
Second entity (if needed):
#Entity(tableName = "user_roles")
data class UserRole(
val user_id: Int,
val role_name: String
)
DAO
#Dao
interface UserDao {
#Query("SELECT * FROM users")
fun getAllUsers(): Flow<List<User>>
}
This will not compile because SQLite doesn't support collections. Is there any way to map the role_name from user_roles to roles property of User entity? Can Room do it automatically?

Answer 1

I don't think Room has automatic support for that requirement. Instead you could do a custom query to achieve that.
Refer to this example of how to do one, and adapt according to your requirements.
getAllUsersWithRoles returns flow of list of UserWithRoles. Query joins users and user_roles tables and maps results to UserWithRolesResult class.
fromResult takes list of UserWithRolesResult and maps it to list of UserWithRoles
#Dao
interface UserDao {
#Transaction
#Query("SELECT * FROM users")
fun getAllUsersWithRoles(): Flow<List<UserWithRoles>>
companion object {
private fun fromResult(result: List<UserWithRolesResult>) : List<UserWithRoles> {
val map = result.groupBy { it.userId }.mapValues { it.value.map { it.roleName }.toSet() }
return map.map { UserWithRoles(it.key, it.value) }
}
}
}
data class UserWithRoles(
val userId: Int,
val roles: Set<String>
)
data class UserWithRolesResult(
val userId: Int,
val roleName: String
)

Answer 2

This will not compile because SQLite doesn't support collections.
Room however does.
Consider the following working Demo based upon your code
note the comments and modifications
run on the main thread for brevity
the POJO UserWithRoles is the main inclusion that uses a Set, which is populated/used
:-
#Entity(tableName = "users")
data class User(
#PrimaryKey
val id: Int,
val name: String
/* roles would be effectively trying to store what is stored elsewhere */
//val roles: Set<String> // want it from role_name column !! Wrong place see UserWithRoles
)
#Entity(
tableName = "user_roles",
foreignKeys = [
ForeignKey(
entity = User::class,
parentColumns = ["id"],
childColumns = ["user_id"]
)
]
)
data class UserRole(
#PrimaryKey
/* In Room and Entity MUST have a primary key */
/* if user_id were a primary key then many UserRoles per User would not be allowed as
a primary key is implicitly UNIQUE. Hence dummy_id
*/
val dummy_id: Long?=null,
val user_id: Int,
val role_name: String
)
data class UserWithRoles(
#Embedded
val user: User,
#Relation(entity = UserRole::class, parentColumn = "id", entityColumn = "user_id" )
val roles: Set<UserRole>
)
#Dao
interface UserDao {
#Insert(onConflict = OnConflictStrategy.IGNORE)
fun insert(user: User): Long
#Insert(onConflict = OnConflictStrategy.IGNORE)
fun insert(userRole: UserRole): Long
#Query("SELECT * FROM users")
fun getAllUsers(): /*Flow<*/List<UserWithRoles>/*>*/
}
#Database(entities = [User::class,UserRole::class], exportSchema = false, version = 1)
abstract class TheDatabase: RoomDatabase() {
abstract fun getUserDao(): UserDao
companion object {
private var instance: TheDatabase?=null
fun getInstance(context: Context): TheDatabase {
if (instance==null) {
instance=Room.databaseBuilder(context,TheDatabase::class.java,"the_database.db")
.allowMainThreadQueries() /* for brevity of demo */
.build()
}
return instance as TheDatabase
}
}
}
and some Activity Code to demonstrate:-
class MainActivity : AppCompatActivity() {
lateinit var db: TheDatabase
lateinit var dao: UserDao
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
db = TheDatabase.getInstance(this)
dao = db.getUserDao()
dao.insert(User(10,"Fred"))
dao.insert(User(11,"Mary"))
dao.insert(User(12,"Jane"))
dao.insert(UserRole(user_id = 10, role_name = "Role1"))
dao.insert(UserRole(user_id = 10, role_name = "Role2"))
dao.insert(UserRole(user_id = 10, role_name = "Role3"))
dao.insert(UserRole(user_id = 11, role_name = "Role4"))
for (uwr in dao.getAllUsers()) {
val sb = StringBuilder()
for (r in uwr.roles) {
sb.append("\n\tRole is ").append(r.role_name)
}
Log.d("DBINFO","User is ${uwr.user.name} and has ${uwr.roles.size} roles. They are ${sb}")
}
}
}
When run then the log includes:-
D/DBINFO: User is Fred and has 3 roles. They are
Role is Role3
Role is Role2
Role is Role1
D/DBINFO: User is Mary and has 1 roles. They are
Role is Role4
D/DBINFO: User is Jane and has 0 roles. They are