I have a custom Membership object that has a master-detail relationship with Account, and a lookup relationship with Contact (an Account owns the membership and a Contact uses it). I have a list of Membership objects on the Account page as well as the Contact page, so that a user can edit a membership from either location.

The question I have concerns when a user adds a new Membership object from either the Account or Contact page. When they add a new instance from Account, SF automatically fills in the Account name. That's fine. From the Contact page, it fills in the Contact name. Also fine.

But from the Contact page, I'd also like the Contact's Account be the default for the Account field, but it is blank. A Contact only has one Account, so it is unambiguous which Account should be used.

But I can't find a way to populate the Account field on the New Membership page. A trigger won't work, since those fire when the user tries to save the new Membership object. I looked to set the default value for the Account field, but that isn't an option for master-details with standard objects.

How can I do this? Is an s-control my only option?

Thanks for any suggestions or pointers!
Don