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?

<%= 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?

All cities Stockholm Göteborg <---- for example if this was the choice ...and so on