_form.erb

<div class="group">
  <%= f.label :name %><%= f.error_message_on :name %>
  <%= f.text_field :name, :class => :text_field %>
  <span class="description">Ex: a simple text</span>
</div>
<div class="group">
  <%= f.label :url %><%= f.error_message_on :url %>
  <%= f.text_field :url, :class => :text_field %>
  <span class="description">Ex: a simple text</span>
</div>

<div class="group">
  <% fields_for  @sponsor.sites do |site| %>
    <%= site.label :name %><%= site.error_message_on :name %>
    <%= site.text_field :name %>
    <%= site.text_field :url %>
  <% end %>
</div>

<div class="group navform wat-cf">
  <%= f.submit pat(:save), :class => :button %>
  <%= f.submit pat(:cancel), :onclick => "window.location='#{url(:sponsors, :index)}';return false", :class => :button %>
</div>

controller

put :update, :with => :id do
    @sponsor = Sponsor.find(params[:id])
    if @sponsor.update_attributes(params[:sponsor])
      flash[:notice] = 'Sponsor was successfully updated.'
      redirect url(:sponsors, :edit, :id => @sponsor.id)
    else
      render 'sponsors/edit'
    end
  end

HTML Result

<form class="form" method="post" action="/admin/sponsors/update/4c0c3c6dd478e11b63000003"><input type="hidden" value="put" name="_method" /> 
        <div class="group"> 
  <label for="sponsor_name">Name: </label> 
  <input type="text" value="test" class="text_field" name="sponsor[name]" id="sponsor_name" /> 
  <span class="description">Ex: a simple text</span> 
</div> 
<div class="group"> 
  <label for="sponsor_url">Url: </label> 
  <input type="text" value="test" class="text_field" name="sponsor[url]" id="sponsor_url" /> 
  <span class="description">Ex: a simple text</span> 
</div> 
 
<div class="group"> 
  
    <label for="array_name">Name: </label> 
    <input type="text" value="" name="array[name]" id="array_name" /> 
    <input type="text" value="" name="array[url]" id="array_url" /> 
  
</div> 
 
<div class="group navform wat-cf"> 
  <input type="submit" value="Save" class="button" /> 
  <input type="submit" value="Cancel" class="button" onclick="window.location='/admin/sponsors';return false" /> 
</div> 
      </form>