Rails link_to destroy a nested resource?

I have a nested resource attachments and I want to create a link_to to destroy/delete the attachment. Here's what I have, but it is posting as a GET versus a PUT:

<%= link_to "Delete Attachment", project_thread_attachment_path(@attachment.thread.project.id, @attachment.thread.id, @attachment.id), :confirm => "Are you sure you want to delete this attachment?", :method => :delete, :action => "destroy" %>

Ideas? Thanks!