field_for

阅读api是一种乐趣！
api of fields_for===========
fields_for(record_or_name_or_array, *args) {|builder.new(object_name, object, self, options, block)| ...}

Creates a scope around a specific model object like form_for, but doesn‘t create the form tags themselves. This makes fields_for suitable for specifying additional model objects in the same form.
Generic Examples

  <% form_for @person, :url => { :action => "update" } do |person_form| %>
    First name: <%= person_form.text_field :first_name %>
    Last name : <%= person_form.text_field :last_name %>

    <% fields_for @person.permission do |permission_fields| %>
      Admin?  : <%= permission_fields.check_box :admin %>
    <% end %>
  <% end %>

…or if you have an object that needs to be represented as a different parameter, like a Client that acts as a Person:

  <% fields_for :person, @client do |permission_fields| %>
    Admin?: <%= permission_fields.check_box :admin %>
  <% end %>

…or if you don‘t have an object, just a name of the parameter:

  <% fields_for :person do |permission_fields| %>
    Admin?: <%= permission_fields.check_box :admin %>
  <% end %>

Note: This also works for the methods in FormOptionHelper and DateHelper that are designed to work with an object as base, like FormOptionHelper#collection_select and DateHelper#datetime_select.
Nested Attributes Examples

When the object belonging to the current scope has a nested attribute writer for a certain attribute, fields_for will yield a new scope for that attribute. This allows you to create forms that set or change the attributes of a parent object and its associations in one go.

Nested attribute writers are normal setter methods named after an association. The most common way of defining these writers is either with accepts_nested_attributes_for in a model definition or by defining a method with the proper name. For example: the attribute writer for the association :address is called address_attributes=.

Whether a one-to-one or one-to-many style form builder will be yielded depends on whether the normal reader method returns a single object or an array of objects.
One-to-one

Consider a Person class which returns a single Address from the address reader method and responds to the address_attributes= writer method:

  class Person
    def address
      @address
    end

    def address_attributes=(attributes)
      # Process the attributes hash
    end
  end

This model can now be used with a nested fields_for, like so:

  <% form_for @person, :url => { :action => "update" } do |person_form| %>
    ...
    <% person_form.fields_for :address do |address_fields| %>
      Street  : <%= address_fields.text_field :street %>
      Zip code: <%= address_fields.text_field :zip_code %>
    <% end %>
  <% end %>

When address is already an association on a Person you can use accepts_nested_attributes_for to define the writer method for you:

  class Person < ActiveRecord::Base
    has_one :address
    accepts_nested_attributes_for :address
  end

If you want to destroy the associated model through the form, you have to enable it first using the :allow_destroy option for accepts_nested_attributes_for:

  class Person < ActiveRecord::Base
    has_one :address
    accepts_nested_attributes_for :address, :allow_destroy => true
  end

Now, when you use a form element with the _delete parameter, with a value that evaluates to true, you will destroy the associated model (eg. 1, ‘1’, true, or ‘true’):

  <% form_for @person, :url => { :action => "update" } do |person_form| %>
    ...
    <% person_form.fields_for :address do |address_fields| %>
      ...
      Delete: <%= address_fields.check_box :_delete %>
    <% end %>
  <% end %>

One-to-many

Consider a Person class which returns an array of Project instances from the projects reader method and responds to the projects_attributes= writer method:

  class Person
    def projects
      [@project1, @project2]
    end

    def projects_attributes=(attributes)
      # Process the attributes hash
    end
  end

This model can now be used with a nested fields_for. The block given to the nested fields_for call will be repeated for each instance in the collection:

  <% form_for @person, :url => { :action => "update" } do |person_form| %>
    ...
    <% person_form.fields_for :projects do |project_fields| %>
      <% if project_fields.object.active? %>
        Name: <%= project_fields.text_field :name %>
      <% end %>
    <% end %>
  <% end %>

It‘s also possible to specify the instance to be used:

  <% form_for @person, :url => { :action => "update" } do |person_form| %>
    ...
    <% @person.projects.each do |project| %>
      <% if project.active? %>
        <% person_form.fields_for :projects, project do |project_fields| %>
          Name: <%= project_fields.text_field :name %>
        <% end %>
      <% end %>
    <% end %>
  <% end %>

When projects is already an association on Person you can use accepts_nested_attributes_for to define the writer method for you:

  class Person < ActiveRecord::Base
    has_many :projects
    accepts_nested_attributes_for :projects
  end

If you want to destroy any of the associated models through the form, you have to enable it first using the :allow_destroy option for accepts_nested_attributes_for:

  class Person < ActiveRecord::Base
    has_many :projects
    accepts_nested_attributes_for :projects, :allow_destroy => true
  end

This will allow you to specify which models to destroy in the attributes hash by adding a form element for the _delete parameter with a value that evaluates to true (eg. 1, ‘1’, true, or ‘true’):

  <% form_for @person, :url => { :action => "update" } do |person_form| %>
    ...
    <% person_form.fields_for :projects do |project_fields| %>
      Delete: <%= project_fields.check_box :_delete %>
    <% end %>
  <% end %>

   # File vendor/rails/actionpack/lib/action_view/helpers/form_helper.rb, line 476
476:       def fields_for(record_or_name_or_array, *args, &block)
477:         raise ArgumentError, "Missing block" unless block_given?
478:         options = args.extract_options!
479: 
480:         case record_or_name_or_array
481:         when String, Symbol
482:           object_name = record_or_name_or_array
483:           object = args.first
484:         else
485:           object = record_or_name_or_array
486:           object_name = ActionController::RecordIdentifier.singular_class_name(object)
487:         end
488: 
489:         builder = options[:builder] || ActionView::Base.default_form_builder
490:         yield builder.new(object_name, object, self, options, block)
491:       end