Kotlin Exposed: Proper way to create a One To Many Relationship? - sql

Question

I want to create an One-To-Many Relationship from the Order Entity to the OrderProductAmount Entity.
I need this, because for each Order I need to know which Product's it contains and what the amount of each Product in the Order is, as one order can contain multiple products.
When I fetch Order Entities from the database, I want to be able to access all rows from OrderProductAmount that contain the corresponding orderId from it.
But whenever I access order.products from the result of findOrder I get null as the result.
I suppose there is some mistake in my setup of the entities or I am not inserting the entities into the database in the right way.
Order Entity
class Order(id: EntityID<Int>) : IntEntity(id) {
companion object : IntEntityClass<Order>(Orders)
var orderStatus by Orders.orderStatus
val products by OrderProductAmount referrersOn OrderProductAmounts.order
}
object Orders : IntIdTable() {
var orderStatus = enumeration("orderStatus", OrderStatus::class)
}
OrderProductAmount Entity
class OrderProductAmount(id: EntityID<Int>) : IntEntity(id) {
companion object : IntEntityClass<OrderProductAmount>(OrderProductAmounts)
var order by Order referencedOn OrderProductAmounts.order
var product by OrderProductAmounts.product
var amount by OrderProductAmounts.amount
}
object OrderProductAmounts : IntIdTable() {
var order = reference("order", Orders)
var product = integer("product")
var amount = integer("amount")
}
Product Entity
class Product(id: EntityID<Int>) : IntEntity(id) {
companion object : IntEntityClass<Product>(Products)
var name by Products.name
var price by Products.price
}
object Products : IntIdTable() {
val name = varchar("name", length = 256)
val price = decimal("price", precision = 10, scale = 2)
}
Inserting new entities ...
override suspend fun addOrder(productIdToAmount: Map<Int, Int>) = dbQuery {
val orderId = Orders.insertAndGetId {
it[Orders.orderStatus] = OrderStatus.CREATED
}
productIdToAmount.entries.forEach { (productId, amount) ->
val product = productDAO.findProduct(productId)!!
OrderProductAmounts.insert {
it[OrderProductAmounts.order] = orderId.value
it[OrderProductAmounts.product] = product.id.value
it[OrderProductAmounts.amount] = amount
}
}
}
Fetching an order from the database ...
override suspend fun findOrder(id: Int): Order? = dbQuery {
Orders.select { Orders.id eq id }.map { Order.wrapRow(it) }.singleOrNull()
}

Answer 1

It seems that your mapping for products is a bit off.
It should be:
val products by Product via OrderProductAmounts
First comes the type of the collection, then the connection table.