editing multiple records at once

I want to be able to change a property on multiple records in my app. I
thought about adding a checkbox so that one can pick which records to
update. The question is how to implement the actual update - if the
checkbox isn't part of the model, how can I iterate the records in the
view and see which are selected? My records are displayed using a
partial:

# _product.html.erb

<%= check_box_tag 'modify' %>
<%=h product.name %>
<%=h product.price %>

# index.html.erb

<% form_tag %>

<%= text_field_tag 'price' %>
<%= render :partial => 'product', @collection => @products %>
<%= submit_tag 'save' %>

<% end %>

Also, what kind of form should I put this in, and how to catch which
boxes are selected... I'm pretty clueless about this - thanks.