How to set Query Parameters in Rails form_for URLs

In Rails 3.2.8, the following code fragment:

  <%= form_for @resource, url: {p: '123'} do |f| %>

generates this HTML:

  <form accept-charset="UTF-8" action="/resource/new?p=123" ... >

Multiple query parameters also work:

  <%= form_for @resource, url: {p: '123', q: 'ABC'} do |f| %>

gives:

  <form accept-charset="UTF-8" action="/resource/new?p=123&q=ABC" ... >