Nested forms issue - rails

Nested forms issue - rails

I have some rails nested forms
= form_for @model do |f|
...
= f.fields_for :user_partnership do |builder|
- users.each do |index|
= builder.hidden_field :user_partner_id
= content_tag(:li,index.name)
In my model I have set has_many :user_partner, through: :users_partnership.
So now I have two problems : the first, I have no input inside the
fields_for process. So, following some SO posts, I had to add this
@model = model.new
@user_partners = User_Partner.all
@user_partners.count.times{@model.users_partnership.build}
but this really seam awkward. The secound question is how I get the
enumerator of the fields_for block so I can set correctly index.name ?
Edit, here's my models
class model
has_many :user_partners, through: :users_partnerships
accepts_nested_attributes_for :users_partnerships
end
class User_Partner
has_many :models, through: :users_partnerships
end
class Users_Partnership
belongs_to :model
belongs_to :user_partner
end