multiple foreign keys on single field, how to best go about?

[Paul Carey]

Hello and thanks for looking,

I'll explain what it is I'm trying to achieve with an example...

I'm trying to record documents that have been produced like the following..

Invoice
invoice_id
...
...

Order
order_id
...
...

DocumentProduced
doc_id
date_printed
type <- A ref to the type of document produced e.g. Quote (1) or Invoice (2), etc
ref_id <- The id of the document e.g. the invoice or quote id

What I'd like to do is set up foreign keys to the tables such as Quote and Invoice on the ref_id field of DocumentProduced as far as I know its only possible to set one foreign key on a field.

Would there be any other way if this is true that it can be done such as constraints or triggers?

Thanks very much in advance

Paul