>I'm guessing your application assumed the FK relationship then and added it behind the scenes.

I checked in the Invoice table and nothing has been added in the Constraints section.

>In any case, you need to add constraints, otherwise you'll end up with orphan records (an you probably already have them).

We are not there yet. We simply want to migrate our data, which is what we did and we need to avoid that kind of verifications by SQL Server because this application has run ok for a few years now.