How can I deal with collections in fieldset?

Hi all,

I have a very complex model which has several has_one and has_many relationships with other models. For the has_one relations, I can use "field_for" to create/update the model information in a single form. But how can I deal with the has_many relations? I know that if I what to have collections in a form, the following code will work:

<% form_tag do %>   <% for @product in @products %>     <%= text_field("product", 'image_url') %><br />   <% end %>   <%= submit_tag %> <% end %>

But how can I do this using "fields_for"? Thanks a lot.