Random Dev Notes (Room ~@Relation~ Dao Error)

Apparently with Room on Android, you can’t use @Insert Dao methods with a POJO. It throws an unhelpful error that took two entire days to figure out. I’m writing this so I never forget and to maybe help someone out who googles it next.

Say you have a model and POJO like:

    @Entity(...)
    data class User(
      val id: String,
      val name: String
    )
    
    @Entity(...)
    data class Pet(
      val id: String,
      val type: String,
      val name: String,
      val userId: String
    )
    
    data class UserWithPets(
      val id: String,
      val name: String,
      @Relation(
        parentColumn = "id",
        entityColumn = "userId"
      )
      val pets: List<Pet>
    )

If you write a Dao method like:

    @Insert(entity = User)
    fun save(userWithPets: UserWithPets)

You will get the error: Partial entities cannot have relations.

If you do a web search, all you get is some source code (line 110) which is hardly helpful. It has a // TODO: Support Pojos with relations.. Maybe this will get added soon?