how to insert value to the extra field in join model

hi all,
         consider that I'm having 3 models
1)user
2)doc
3)udoc

user model=>(id,name)
has_many:udocs
has_many:docs ,through:udocs

doc Model=>(id,name)
has_many:udocs
has_many:users ,through:udocs

udoc Model=>(id,user_id,doc_id,start_date,end_date)
belongs_to:user
belongs_to;doc

Can anybody just tell how to access the extra field (start_date and
end_date) in udocs table
by using User Model

Assuming you have a user in, say, @user, then @user.udocs will give
you an array of Udoc objects so you can do things like
@user.udocs[0].start_date
or
@user.udocs.each do |udoc|
# do something with udoc.end_date
end

Colin