Just for reference...

class Member < ActiveRecord::Base
    has_one :member_profile
    accepts_nested_attributes_for :member_profile
end

<% form_for @member do |f| -%>
       <%= f.label :fullname %> <%= f.text_field :fullname %>
    <% f.fields_for :member_profile, @member.member_profile do |member_profile|%>
               
<%= member_profile.label :about %><%= member_profile.text_field :about %>
               
<%= member_profile.label :favorite_color %><%= member_profile.text_field :favorite_color %>
    <% end %>