fields_for can not group its params when ajax

  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"=>""},

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

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

new_book page:

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

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

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