Add Data in related tables at the same time in LightSwitch

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C#

LightSwitch

When i adding new Product and ProductDetails at the same time in a screen I have a problem with Computed Properties in ProductDetails Entity Because There is no ProductID yet and other ID's that are Foreign key in ProductDetails was not selected yet so Computed Properties in ProductDetails such as AutoName(that Computed based on ProductProperties),CategoryName, ... face to exception.
how we can handle these 2 Problem:

1) There is no ProductID as a Parent yet
2) how to select set Of property for a product as a ProductDetails by selecting Productkind when click on add button in ProductDetails child screen

Actually we want to show a modal window to select category before add a ProductProperty to a new Product, to add related Properties automaticlly at ProductDetails Grid, how can we do now?