Passing an array into hidden_field ROR
I would use this technique.
<% @user.roles.each do |role| %> <%= f.hidden_field :role_ids, :multiple => true, :value => role.id %><% end %>:multiple adds both the [] to the end of the name attribute and multiple="multiple" to the input element, which is ideal. Rails uses this internally when it generates inputs for arrays, such as in fields_for.
Unfortunately it is not well-documented. I'm going to look into fixing this.
The only thing that works for me (Rails 3.1) is using hidden_field_tag:
<% @users.roles.each do |role| %> <%= hidden_field_tag "user_role_ids[]", role.id %><% end %>
Try:
<% @user.roles.each_with_index do |role| %> <%= f.hidden_field "role_ids[]", :value => role.id %> <% end %>