Also how about the following approach:

Document doc = new Document();
doc.Persist(new CsvPersister())

and in the Persist of the Document:

public void Persist(IPersister persister)
{
   persister.Persist(this);
}

Hi Gabriel

I think this crops up in many similar scenarios, for example users and
roles. Does a User object maintain a list of roles to which it belongs, or
does a Role object maintain a list of Users who have that role, or both.

In your scenario, I would say that if your Document object implements some
common (to your application) interface, such as IPersistable, then solution
2 would work well. Your CsvPersister would not need to understand how to
persist a document, only how to persist an object that implements
IPersistable. It also implies that you have a common format that you would
use for a variety of persistable objects.

On the other hand, if you only have one or two objects that will be
persisted, and each in a different way, then I would promote solution 1.

HTH

Charles