Updating data from a relation

Hello i got this:

Ex:

Member
   - name
   - phone
   - rel_member_id

rel_members
  - id
  - member_id
  - membership_id

Memberships
  - id
  - name
  - another_data

Now in my edit.rhtml.erb
i got al the stuff...

and this: <%=select 'rel_members','membership_id',
@objectWithTheCollect%>

my models are connected really good, but i know that something is wrong
in the way i am trying to update the membership of the member.

some help please