帮助渲染来自fields_for的部分内容(Help to render a partial from fields_for)

 我正在使用nested_attributes并试图通过Ryan Bates关于嵌套模型（＃196）的截屏实现添加/删除字段， 
 
 这样做，它不会工作，但是当删除“link_to_add_fields”行时，它工作正常。 
 
 问题是我不知道我是否正确地完成了所有的关联。 
 
<%= form_for(@item) do |f| %>
<% if @item.errors.any? %>
<div id="error_explanation">
  <h2><%= pluralize(@item.errors.count, "error") %> prohibited this item from being saved:</h2>

  <ul>
  <% @item.errors.full_messages.each do |msg| %>
    <li><%= msg %></li>
  <% end %>
  </ul>
  </div>
  <% end %>

  <div class="field">
    <%= f.label :title %><br />
    <%= f.text_field :title %>
  </div>
  <div class="field">
    <%= f.label :item_type_id %><br />
    <%= f.select :item_type_id, @item_types.collect { |p| [ p.title, p.id ]} %>
    <br />
    <%= f.fields_for :item_parts do |parts_form| %>
      <%= render "item_part_fields", :p => parts_form %>
    <% end %>
  </div>
  <%= link_to_add_fields "Add Part", f, :part %>
  <div class="actions">
    <%= button_for "Save Item", :class => 'positive pill button', :button_type => 'submit' %>
  </div>
<% end %>
 
 其实我的模特是： 
 
 “ItemPart”模型： 
 
class ItemPart < ActiveRecord::Base
  belongs_to :item
  belongs_to :part 
end
 
 “物品”型号： 
 
class Item < ActiveRecord::Base
  belongs_to :item_type
  has_many :item_parts
  has_many :parts, :through => :item_parts
  accepts_nested_attributes_for :item_parts, :allow_destroy => true
  after_save :save_id
  validates :title, :presence => true

  def save_id
    item_part_attributes = [ { :item_id => self.id } ]
  end

end
 
 “部分”模型： 
 
class Part < ActiveRecord::Base
  has_one :part_type
  has_many :item_parts
  has_many :items, :through => :item_parts

end
 
 我正在这样做的错误： 
 
undefined method `klass' for nil:NilClass
Extracted source (around line #26):



23:       <%= render "item_part_fields", :p => parts_form %>
24:     <% end %>
25:   </div>
26:   <%= link_to_add_fields "Add Part", f, :item %>
27:   <div class="actions">
 
 应用跟踪 
 
app/helpers/application_helper.rb:44:in `link_to_add_fields'
app/views/items/_form.html.erb:26:in `block in _app_views_items__form_html_erb__1418129287024756954_2169222480__3228169100620818569'
app/views/items/_form.html.erb:1:in `_app_views_items__form_html_erb__1418129287024756954_2169222480__3228169100620818569'
app/views/items/edit.html.erb:3:in `_app_views_items_edit_html_erb___1857878264245794505_2169270380_890290209246324491'
 
 Application_helper.rb 
 
def link_to_add_fields(name, f, association)
  new_object = f.object.class.reflect_on_association(association).klass.new # error line :44
  fields = f.fields_for(association, new_object, :child_index => "new_#{association}") do |builder|
    render(association.to_s.singularize + "_fields", :f => builder)
  end
  link_to_function(name, "add_fields(this, '#{association}', '#{escape_javascript(fields)}')" )
end

I'm using nested_attributes and trying to implement the add/remove fields on-the-fly throu ajax following Ryan Bates screencast about Nested Model (#196)
 
Doing this, it won't work, but when removing the "link_to_add_fields" line, it works fine.
 
The problem is that I'm don't know if I doing this all associations right.
 
<%= form_for(@item) do |f| %>
<% if @item.errors.any? %>
<div id="error_explanation">
  <h2><%= pluralize(@item.errors.count, "error") %> prohibited this item from being saved:</h2>

  <ul>
  <% @item.errors.full_messages.each do |msg| %>
    <li><%= msg %></li>
  <% end %>
  </ul>
  </div>
  <% end %>

  <div class="field">
    <%= f.label :title %><br />
    <%= f.text_field :title %>
  </div>
  <div class="field">
    <%= f.label :item_type_id %><br />
    <%= f.select :item_type_id, @item_types.collect { |p| [ p.title, p.id ]} %>
    <br />
    <%= f.fields_for :item_parts do |parts_form| %>
      <%= render "item_part_fields", :p => parts_form %>
    <% end %>
  </div>
  <%= link_to_add_fields "Add Part", f, :part %>
  <div class="actions">
    <%= button_for "Save Item", :class => 'positive pill button', :button_type => 'submit' %>
  </div>
<% end %>
 
Actually my models are:
 
"ItemPart" model:
 
class ItemPart < ActiveRecord::Base
  belongs_to :item
  belongs_to :part 
end
 
"Item" model:
 
class Item < ActiveRecord::Base
  belongs_to :item_type
  has_many :item_parts
  has_many :parts, :through => :item_parts
  accepts_nested_attributes_for :item_parts, :allow_destroy => true
  after_save :save_id
  validates :title, :presence => true

  def save_id
    item_part_attributes = [ { :item_id => self.id } ]
  end

end
 
"Part" model:
 
class Part < ActiveRecord::Base
  has_one :part_type
  has_many :item_parts
  has_many :items, :through => :item_parts

end
 
The error I'm getting doing this way:
 
undefined method `klass' for nil:NilClass
Extracted source (around line #26):



23:       <%= render "item_part_fields", :p => parts_form %>
24:     <% end %>
25:   </div>
26:   <%= link_to_add_fields "Add Part", f, :item %>
27:   <div class="actions">
 
Application trace
 
app/helpers/application_helper.rb:44:in `link_to_add_fields'
app/views/items/_form.html.erb:26:in `block in _app_views_items__form_html_erb__1418129287024756954_2169222480__3228169100620818569'
app/views/items/_form.html.erb:1:in `_app_views_items__form_html_erb__1418129287024756954_2169222480__3228169100620818569'
app/views/items/edit.html.erb:3:in `_app_views_items_edit_html_erb___1857878264245794505_2169270380_890290209246324491'
 
Application_helper.rb
 
def link_to_add_fields(name, f, association)
  new_object = f.object.class.reflect_on_association(association).klass.new # error line :44
  fields = f.fields_for(association, new_object, :child_index => "new_#{association}") do |builder|
    render(association.to_s.singularize + "_fields", :f => builder)
  end
  link_to_function(name, "add_fields(this, '#{association}', '#{escape_javascript(fields)}')" )
end

原文：https://stackoverflow.com/questions/6939634