fields_for can not group its params when ajax

hi   book and page is one_to_many relationship.I want to create them in a single form.But when clicking create button,the submitted params for pages are not grouped correctly, i want "book"=>{"new_pages"=>[{"name"=>"", "color"=>""}, {"name"=>"","color"=>""}]

but it is "book"=>{"new_pages"=>[{"name"=>""}, {"name"=>"", "color"=>""}, {"color"=>""}]\

My page and params are listed below.please help me,this take me a whole day.

new_book page:

<div><%=error_messages_for :book-%></div> <%form_remote_for :book,@book,:update=>"book{@book.id}",:url=>books_path do>v>%>      <table>        <tr>          <td><%=v.label "name"-%></td>          <td><%=v.text_field :name-%></td>        </tr>      </table>       <div id="pages">        <%=render :partial=>'page',:collection=>@book.pages-%>       </div>      <%=submit_tag 'create'-%> <%end-%>

page partial <%fields_for "book[new_pages]",page do|o|-%>     <table>       <tr>         <td><%=o.label "name"%></td>         <td><%=o.text_field :name%></td>         <td><%=o.label "color"%></td>         <td><%=o.text_field :color%></td>       </tr>     </table> <%end-%>

"book"=>{"new_pages"=>[{"name"=>""}, {"name"=>"", "color"=>""}, {"color"=>""}]...