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

I had a similar issue. Here is my solution.

You can use an extra entity (ReservationGuest) which keeps the relation between Guest and Reservation.

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

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

@Entity data class ReservationGuest(
    @PrimaryKey(autoGenerate = true) val id: Long,
    val reservationId: Long,
    val guestId: Long
)

You can get reservations with their list of guestIds. (Not the guest objects)

data class ReservationWithGuests(
    @Embedded val reservation:Reservation,
    @Relation(
        parentColumn = "id",
        entityColumn = "reservationId",
        entity = ReservationGuest::class,
        projection = "guestId"
    ) val guestIdList: List
)

You can also get guests with their list of reservationIds. (Not the reservation objects)

data class GuestWithReservations(
    @Embedded val guest:Guest,
    @Relation(
        parentColumn = "id",
        entityColumn = "guestId",
        entity = ReservationGuest::class,
        projection = "reservationId"
   ) val reservationIdList: List
)

Since you can get the guestIds and reservationIds, you can query Reservation and Guest entities with those.

I'll update my answer if I find an easy way to fetch Reservation and Guest object list instead of their ids.

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