Entity Relationship question

[Michael Kellemon]

We are in the process of redesigning our datamodel and I have a question around the best way to relate one entity to another.

We have several scenarios where this applies but I will use one for illustration.

Tables as follows

Documents
Users
Clients

A document could could relate to one Client, to one User or to both one User and one Client. So my question is am I best to use a intermediate table to relate them such as
Document_Entity de
de.entity_type
de.entity_id

or add a Document.user_id and a Document.client_id column to the Documents table.

As I see it the advantage to an intermediate table is flexibility and no empty columns.

The advantage of adding columns in the Document table is simplicity and faster joining assuming I have many Documents, Users and Clients. In the past we have opted for the latter and think that is the way to go but any insight would be greatly appreciated.

Thanks