How can I represent a many to many relation with Android Room?

Question

How can I represent a many to many relation with Room? e.g. I have \"Guest\" and \"Reservation\". Reservation can have many Guest and a Guest can be part of many Reservation

Answer 1

With the introduction to Junction in room you can handle many-to-many relationship with ease.

As @Devrim stated you can use an extra entity (ReservationGuest) which keeps the relation between Guest and Reservation(also know as associative table or junction table or join table).

@Entity
data class Guest(
  @PrimaryKey
  val gId: Long,
  val name: String,
  val email: String
)

@Entity
data class Reservation(
  @PrimaryKey
  val rId: Long,
  val table: String
)

@Entity(
  primaryKeys = ["reservationId", "guestId"]
)
data class ReservationGuest(     
  val reservationId: Long,
  val guestId: Long
)

Now you can get reservation with guests using this model:

data class ReservationWithGuests (
    @Embedded
    val reservation: Reservation,
    @Relation(
            parentColumn = "rId",
            entity = Guest::class,
            entityColumn = "gId",
            associateBy = Junction(
                    value = ReservationGuest::class,
                    parentColumn = "reservationId",
                    entityColumn = "guestId"
            )
    )
    val guests: List
)

You can also get guest with their list of reservations as.

data class GuestWithReservations (
  @Embedded
  val guest: Guest,
  @Relation(
        parentColumn = "gId",
        entity = Reservation::class,
        entityColumn = "rId",
        associateBy = Junction(
                value = ReservationGuest::class,
                parentColumn = "guestId",
                entityColumn = "reservationId"
        )
  )
  val reservations: List
)

Now you can query database for the result as:

@Dao
interface GuestReservationDao {
  @Query("SELECT * FROM Reservation")
  fun getReservationWithGuests(): LiveData>

  @Query("SELECT * FROM Guest")
  fun getGuestWithReservations(): LiveData>

}