Nested Forms adding [-]

Im stumped. I followed Ryan Bates screencast 73 and had this working
the other day for an assets model. Today im trying to implement the
same thing for a case_study model that belongs_to :services.
MODEL: CaseStudy
belongs_to :services

MODEL: Services
has_many :case_studies, :dependent => :destroy

  def case_studies=(case_studies)
    case_studies.each do |attribute|
      case_studies.build(attribute)
    end
  end

CONTROLLER: new method
    @service.case_studies.build

in my _form partial for services
<% form_for(@service, :html => {:multipart => true}) do |f| %>
...
(this is the nested part)
      <% for case_study in @service.case_studies %>
        <% fields_for "service[case_studies][]", case_study do |
case_study_form|%>
          <p>
            <%= case_study_form.label :title %><br />
            <%= case_study_form.text_field :title %>
          </p>
          <p>
            <%= case_study_form.label :description %><br />
            <%= case_study_form.text_area :description, :rows =>
5, :cols => 29 %>
          </p>
          <p>
            <%= case_study_form.label :permalink %><br />
            <%= case_study_form.text_field :permalink %>
          </p>
          <p>
            <%= case_study_form.label :keywords %><br />
            <%= case_study_form.text_field :keywords %>
          </p>
          <p>
            <%= case_study_form.label "Add Case Study" %><br />
            <%= case_study_form.file_field :attachment, :size => 16 %>
          </p>
        <% end %>
      <% end %>
...
<% end %>
This is where its weird. I can raise the params and it shows the hash,
but it adds a [-] hash to the case_studies hash.

all the form fields are named service[case_studies][-][attr etc...]
when i view the source

any help here?

Thanks in advance

Bob