Set a Parent property on Child Objects when binding

Question

We have a hierarchical model of the shape (yuml-ized):

And a controller action that takes an Order-typed parameter:

public ActionResult UpdateOrder(Order order)
{
    ...
}

With the posted form having fields for the line items, too, we get a tree of objects from the MVC's model binding where as the binders create a new order object for each order to be bound.

The question: Is it possible to have the binding mechanism set the Parent property of each LineItem to the Order object they are being added too?

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I take it that the binder that binds the Order object is responsible for executing a binder for each child object, too, by calling BindModel on the child object binders. We already have custom implementations (inheriting from DefaultModelBinder) in place: A OrderBinder as well as a LineItem binder.

How can our LineItem.BindModel method find out whether the LineItem being bound is "stand-alone" or about to be added to an Order whose binding is in progress, and if the latter is the case, how do we get the reference to that Order?