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

Based on the answer above: https://stackoverflow.com/a/44428451/4992598 only by keeping separate field names between entities you can have models returned (not just ids). All you need to do is:

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

And yes entities can be embedded in one another as long as you don't keep duplicate fields. So in consequence the ReservationWithGuests class can look like this.

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

So at this point you can use val guestIdList: List because your ReservationGuest entity actually maps ids with entity models.