Can an hidden_field changed be detected by observe_form?

Rails
1

in a view, I am changing programmatically an hidden field ‘category_id’
(simulating a click)

  • <%= link_to_function("Baby-sitting", nil, :class => "enfants") do |page| page <

    • …
    <%= hidden_field_tag :category_id, @category_id -%>

    I try to submit the form whenever this field (or other standard input
    field…) is changed

    <%= observe_form(‘prop_search_form’,
    :loading => “Element.show(‘search_spinner’);
    Form.disable(‘prop_search_form’);”,
    :complete => “Element.hide(‘search_spinner’);
    Form.enable(‘prop_search_form’);”,
    :update => {}, :url => { :controller => ‘proposal’, :action => ‘list’ })
    %>

    any field is correctly detected but not this hidden field… is it wrong
    ?
    I also tried to put this value in a readonly text field… same
    problem… change event is not detected

    thanks for your help