ruby-on-rails – Rails 4：fields_for in fields_for

class Child < ActiveRecord::Base
    belongs_to :father
    accepts_nested_attributes_for :father
end

class Father < ActiveRecord::Base
    has_one :child
    belongs_to :grandfather
    accepts_nested_attributes_for :grandfather
end


class Grandfather < ActiveRecord::Base
    has_one :father
end

我在Railscast上使用嵌套模型表格第1部分来获取以下内容：
在children_controller.rb中：

def new
    @child = Child.new
    father=@child.build_father
    father.build_grandfather
  end

def child_params
      params.require(:child).permit(:name,father_attributes:[:name],grandfather_attributes:[:name])
    end

和我的形式：

<%= form_for(@child) do |f| %>
  <div class="field">
    <%= f.label :name %><br>
    <%= f.text_field :name %>
  </div>
  mother:<br>
  <%= f.fields_for :father do |ff| %>
    <%= ff.label :name %>
    <%= ff.text_field :name %><br>
      grand mother:<br>
      <%= f.fields_for :grandfather do |fff| %>
        <%= fff.label :name %>
        <%= fff.text_field :name %>
      <% end %>
  <% end %>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

我试图通过以下方式检索数据：

<%= child.father.name %>
<%= child.father.grandfather.name %>

但祖父的名字不会奏效.
我找不到错误…有人帮忙吗？
谢谢！