gistfile1.rb

class User < ActiveRecord::Base
end

# doesn't work:

class Item < ActiveRecord::Base
  belongs_to :user

  accepts_nested_attributes_for :user
end

# does work:

class Item < ActiveRecord::Base
  belongs_to :user

  def user_attributes=(attrs)
    @user_attributes = attrs
  end
  
  after_save :update_user_attributes
  
  def update_user_attributes
    user.update_attributes! @user_attributes
  end
end