has_one関連時のaccepts_nested_attributes_for / fields_for

_form.html.haml

= form_for @user do |f|
  = f.fields_for :profile, @user.profile || Profile.new do |p|
    .field
      = p.label :nickname
      = p.text_field :nickname

profile.rb

class Profile < ActiveRecord::Base
  belongs_to :user
end

user.rb

class User < ActiveRecord::Base
  has_one :profile
  accepts_nested_attributes_for :profile
end