has_many :through complex form hint request

hi everyone,
I've seen a lot of examples for complex forms which involves all kinds
of associations (railscasts, app examples on github, etc.), only
has_many :through have been left in the dust a bit.

I'm taking advantage and toying around the new :accessible => true for
mass assignments on new records; however I want to ask if somebody
have an example on how to use properly the handling of
has_many :through associations in edge.

Let's say in my create action of the interested controller I have
this:

def create
...
@report = @document.reports.build(params[:report])
if @report.save
...

Report has_many Reason :through the join table, so in my form partial
I've the classic fields_for which does pass the natural
"report"=>{"when(1i)"=>"2008", "when(2i)"=>"8", "when(3i)"=>"29",
"reasons"=>[{"content"=>"my reason"}]}

Remember that in Report model there's the new :accessible for mass
assignment:
has_many :report_reasons
has_many :reasons, :through => :report_reasons, :accessible => true

This works well and it does keep one clean line in controller for
building up the @report instance.
The problem with has_many :through is that every time the Report gets
saved, it creates new entries for every Reason instead of assigning
one that already exists (of course).

Do you have any hints how to manage cleanly this situation? callbacks,
real code examples, etc. can help me greatly.