title: Unidirectional parent-child association not null
body: <p>I have a class structure which is akin to a PurchaseOrder (parent) and PurchaseOrderLine (child) pattern, where the order lines will only be saved to the DB by saving the parent PurchaseOrder and will only ever be accessed via the parent too.</p>
  
  <p>The DB has PurchaseOrderLine.PurchaseOrder set to not permit null values.</p>
  
  <p>It seems from searching through the web that it is not possible to have a uni-directional association from PurchaseOrder via an IList property without having to have a property on the line pointing back when the child has a NOT NULL constraint for its PurchaseOrder column.</p>
  
  <p>Is this really the case?  It seems like one of the most basic things one would want to do with an ORM, I find it difficult to accept that a product as mature as NHibernate cannot handle such a basic scenario.</p>