When I refreshed the Relational Model Data, one of the relationships that is in the database was removed. The output said: The relationship 'Invoice.RefNo - RefNo.Invoice (m:1)' has been removed because its underlying foreign key constraint wasn't found in the refreshed relational model data.

It turnes out the FRef field in the Invoice table isn't there in the Project Explorer window although it is in the Catalog Explorer.

Any ideas why this might happen?

In common with my previous problem, removing the table and adding it again seems to sort out the problem with the missing field. It hasn't sorted the missing relationship though.

I can add the relationship in manually but it disappears when I refresh it again.

Any ideas?

So i.o.w. there's no FK field in Invoice for this relationship? It should have an FK field for the relationship. (it should have been added automatically).

That's right. The FK field in Invoice exists, you can see it in the project explorer but it, and the relationship, aren't in the Catalog Explorer

It would be best if you attached the project file and pointed out which relationship etc. it's about. If you want to keep it hidden, please open a helpdesk thread and attach it there (please refer to this thread there for clarity )