Can't save same Kotlin data class multiple times with ToMany

Issue Basics

• ObjectBox version (are using the latest version?): 2.0.0
• Reproducibility: always

Reproducing the bug

Description

I'm using ObjectBox to save data. When I save the same data with ToMany, it only save the first one.

I write a test, the size of result.details is 1.

Can I save multiple same data with ToMany? How can I fix it?

Thanks for answer.

ObjectBox(version:2.0.0)
Android(target sdk version:27)
Kotlin(version:1.2.60)

Code

@Test
fun orderDetailTest() {
    val orderBox: Box<Order> = store.boxFor()

    // 儲存兩個內容相同的資料
    val order = Order().apply {
        this.details.add(OrderDetail(productSpecName = "ABC"))
        this.details.add(OrderDetail(productSpecName = "ABC"))
    }

    val orderId = orderBox.put(order)

    val result = orderBox.get(orderId)

    assert(result.details.size == 2)
}

Entities

@Entity
data class Order(@Id var id: Long = 0,
                 val createTime: Long = 0,
                 val total: Int = 0) {

    @Backlink(to = "order")
    lateinit var details: ToMany<OrderDetail>
}

@Entity
data class OrderDetail(@Id var id: Long = 0,
                       val productName: String = "",
                       val productSpecName: String = "",
                       val productSpecPrice: Int = 0,
                       val quantity: Int = 0,
                       val subtotal: Int = 0) {

    lateinit var order: ToOne<Order>
}

Start by reproducing the orderDetailTest case with the shown Order and OrderDetail entities, then inspect how the ToMany backlink and ToOne relation are persisted. Done means saving two identical OrderDetail instances results in result.details.size == 2, or the expected limitation is documented.