select option to a select_tag after submitting form

How can I keep the selected option in the select_tag after submitting a
form to the same page?

Form
<%= select_tag :city_id,
options_from_collection_for_select(City.find(:all), :id, :name) %>

After submit the page renders and then I want the selected city to be
displayed in the select_tag, how can I do this?

<select name="advertisement_country_id" id="advertisement_country_id">
<option value="">All cities</option>
<option value="1">Stockholm</option>
<option value="2" selected="selected">Göteborg</option> <---- for
example if this was the choice
...and so on
</select>

THANKS!