When you drag one field (Invoices.InvoiceID) on top of a second field (InvoiceLines.InvoiceID) you get the message "Do you want to create the relation?"
Is it possible to extend this text with which fields you want to connect?
Do you want to create the relation on
Invoice.InvoiceID and InvoiceLines.InvoiceID
This will give you an extra check that you dropped the first field on the right second field.