来看一个典型的场景，Project这个Model的new表单，我们需要在创建Project对象时也创建一些Task对象

我们可以使用fields_for这个标签：

<!-- new.rhtml -->
<% form_for :project, :url => projects_path do |f| %>
  <p>
    Name: <%= f.text_field :name %>
  </p>
  <% for task in @project.tasks %>
    <% fields_for "project[task_attributes][]", task do |task_form| %>
      <p>
        Task: <%= task_form.text_field :name %>
      </p>
    <% end %>
  <% end %>
  <p><%= submit_tag "Create Project" %></p>
<% end %>

这里我们使用fields_for来为@project的每个task构建一个伪表单，实际上还是@project最外层同一个表单，不过这样我们就
可以在同一表单里添加额外的Model对象字段，再看一个官方的例子:

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

然后我们在Project这个Model里添加task_attributes=方法:

# models/.projects.rb
def task_attributes=(task_attributes)
  task_attributes.each do |attributes|
    tasks.build(attributes)
  end
end

这样，我们的controller里的create方法没有受到任何“污染”:

# projects_controller.rb
def new
  @project = Project.new
  3.times { @project.tasks.build }
end

def create
  @project = Project.new(params[:project)
  if @project.save
    flash[:notice] = "Successfully created project."
    redirect_to projects_path
  else
    render :action => 'new'
  end
end