This is what I went for, is there a cleaner or more idiomatic way to do it?

return transaction {
           //Check if a user with this device id exists       
            val userIdEntity = Users.select {
                Users.deviceId eq deviceId
            }.map { it[Users.id] }.firstOrNull()

            //Assign the user id 
            val userId =
                    if(userIdEntity != null) {
                        userIdEntity.value
                    } else
                    {
                        Users.insertAndGetId {
                        //...
                        }.value
                    }
        }

val userIdEntity = Users.select {
                Users.deviceId eq deviceId
            }.limit(1).singleOrNull()?.get(Users.id]

But your code is fine too.