NeededByEOF0

Hi folks, getting a strange problem.

I have a Study <<--> Site.

I have a form in which I edit an individual Study.
This form has a selection box which contains all available sites, with
study.site bound to the selection binding on the WOPopUpButton. Simple
stuff.

When I edit an existing study, change the Site, and try to save, for some
reason EOF tries to update the Site table as well, and causes a primary key
integrity constraint on the Site table to be triggered. Not only this, but
it generates SQL for which I have no explanation (names changed to protect
the guilty):

	"UPDATE SITE SET ID_SITE = ? WHERE ID_SITE = ?" withBindings:
	1:<blah>(NeededByEOF0),
	2:<blah>(NeededByEOF0)>

What's NeededByEOF0? A search of the archives for NeededByEOF0 yields
problems with flattened relationships and propagates primary key, but there
are no flattened relationships between Study and Site, or any use of
propagates primary key anywhere.

Does anybody have any ideas or clues as to where to start looking to fix
this? Makes not a jot of sense to me.

Thanks
Igor.