Problem with Parent Child Data Binding

I have a parent child relationship, on DocumentID, between an invoice and
invoice lines. A form displays invoice fields and has a grid for invoice
lines. The grids BindingSource points to the Invoice Binding Source, and
it's DataMember to the relation, so I think this is correct.



Now, if I leave the relation type to Only Relation, the DocumentID in my new
Invoice is -1, and the DocumentID in my Invoice Lines grid is also -1, but
the Line record remains -1 after being written to the database. So, I think
maybe I should make the relation a constrain as well and add a cascade
update, but then I get an error that -1 doesn't exist in the parent table,
even though the Invoice datatable DocumentID is -1.