Why are <input> element ID's repeating in nested form?

class User
    has_one :user_profile
end
class UserProfile
    belongs_to :user
end

I am rendering a form for editing using partials.

The views are:

user_profile/edit.html.erb
--------------------------
<%= render 'form开发者_C百科' %>

user_profile/_form.html.erb
---------------------------
<%= form_for @user_profile do |f| %>
  <%= f.fields_for @user_profile do |builder| %>
    <%= render :partial => 'user_profiles/fields', :locals => { :f => builder } %>
    # id is correct i.e. [user_profile][last_name] for the field below 
    <%= f.text_field :last_name %>
  <% end %>
  <%= f.submit %>
<% end %>


user_profile/_fields.html.erb
-----------------------------
<%= f.text_field :first_name %>
<%= f.text_field :last_name %>

# id's for the above fields are rendered as:
# user_profile[user_profile][first_name]
# notice the second repeated [user_profile]

How do I fix it so that the id's come out as user_profile[first_name] instead of user_profile[user_profile][first_name] ?

From the documentation for fields_for it's:

suitable for specifying additional model objects in the same form.

With an example showing this paradigm:

<%= form_for @person do |person_form| %>
  First name: <%= person_form.text_field :first_name %>
  Last name : <%= person_form.text_field :last_name %>

  <%= fields_for @person.permission do |permission_fields| %>
    Admin?  : <%= permission_fields.check_box :admin %>
  <% end %>
<% end %>

Notice form_for @person and fields_for @person.permission - fields_for should be used for an association of a model, form_for should be used for the model itself. The reason your html is rendering as user_profile[user_profile] is that you're specifying @user_profile in both form_for and fields_for.

You likely need to change the parameter in form_for to @user (or the equivalent variable you have in scope). This will make your output html:

user[user_profile][first_name]

which you can pass to UsersController#update as long as User accepts_nested_attributes_for user_profile. If you want to instead pass the parameter hash from the form to UserProfilesController#update, then you want to drop the fields_for entirely and just render the fields under form_for @user_profile - which will give you html like user_profile[first_name].