How to populate back reference while binding nested autogrown list?

[Madkinder]

Preface

I've got a classical Parent-Child problem. Here's the code:

@Entity
public class Supply {
    @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REFRESH, CascadeType.DETACH }, mappedBy = "supply")
    private List<SupplyItem> items = new ArrayList<SupplyItem>();
    
    // Getters/Setters and id field
}

@Entity
public class SupplyItem {
    @NotNull
    @ManyToOne
    private Supply supply;

    private Integer quantity;

    // Getters/Setters and id field
}

When I'm trying to save a bean tree in one go:

Supply supply = new Supply();
SupplyItem item = new SupplyItem();

supply.getItems().add(item);

entityManager.persist(supply);

Naturally I get ConstraintViolationException since I have validation configured and item.supply is null.

Hibernate in Action suggests the following approach:

public class Supply {
    // Skipped
    public void addItem(SupplyItem item) {
        item.setSupply(this);
        items.add(item);
    }
    // Skipped
}

Supply supply = new Supply();
SupplyItem item = new SupplyItem();

supply.addItem(item);

entityManager.persist(supply);

Problem

The problem is that I don't populate items by hand, thus I cannot use a helper addItem() method. Instead I get my supply object constructed via web binding. The items list is populated by means of list autogrowing facility.

So my view markup looks like this:

<form:form modelAttribute="supply">
    <!-- Other supply fields -->
    <form:input path="items[0].quantity" />
    <form:input path="items[1].quantity" />
</form:form>

I know that I could iterate over all the supply items and set the back reference to supply, but that looks like a hack to me.

So I wonder is there any other way to make binder/expression parser/whatever to do that for me ? Maybe I could provide the factory method for supply items creation somehow?

If that matters I use WebFlow, but I would like to know whether it is possible for MVC too.

[Madkinder]

Ok, some digging shown that auto growing is implemented in spring-expression (see Indexer.java:285 and Indexer.java:288) which is used by both MVC and WebFlow. So the behavior is consistent, and that's a good thing to know.

What's bad is that new instances are created via Class.newInstance() rather then using a factory, and thus the instantiation process cannot be controlled .

Are there any other options I could cope with my problem?