select statements

Any ideas on this would be great, I'm trying to edit some drop down
lists, the collection _select below does everthing I want it to do and
updates the database successfully but doesn't show the default value
of the drop down when it appears

<%=collection_select('assigned', :qual_id,
@quals,:id, :screen_qual)%>

Because of this i started to play with the select statement below
instead, this causes the drop downs to display properly with the
correct stored values

options_from_collection_for_select(@quals, :id, :screen_qual,

but when i try to submit causes an error

'We're sorry, but something went wrong.
We've been notified about this issue and we'll take a look at it

Am i doing somehtinf obviously wrong? Assigned is a join table between
Degree and Qual, the edit function is on a single degree page.