Peter Marklund

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Rails Workaround: Preserving Nested Hash Params With List Values Across Requests

Suppose you have an HTML search form with a multi-valued select with the name "person[personal][interests][]". The trailing brackets are there to indicate to rails that this HTTP parameter should be converted to a Ruby list. Now, suppose the form submits to the search action in the contacts controller. Our params hash will then contain:

    params = {
      :controller => 'contacts',
      :action => 'search',
      :person => {
        :personal => {
          :interests => ['music', 'chess']

So far so good. Now suppose you have pagination links on the search results page and also that you want to provide a link or button back to the search form for refining the search criteria. Here come the bad news - url_for which is used to create links doesn't support nested hash parameters and also doesn't support list values (there are some patches already to address at least the nesting problem). Here is what url_for produces in this case:

    > puts CGI.unescape(url_for(params))
    > /contacts/search/?person=personalinterestsmusicchess

Clearly url_for is inadequate in this case. I have written a set of helper methods to help save the day:

  def flatten_hash(hash = params, ancestor_names = [])
    flat_hash = {}
    hash.each do |k, v|
      names =
      names << k
      if v.is_a?(Hash)
        flat_hash.merge!(flatten_hash(v, names))
        key = flat_hash_key(names)
        key += "[]" if v.is_a?(Array)
        flat_hash[key] = v
  def flat_hash_key(names)
    names =
    name = names.shift.to_s.dup 
    names.each do |n|
      name << "[#{n}]"
  def hash_as_hidden_fields(hash = params)
    hidden_fields = []
    flatten_hash(hash).each do |name, value|
      value = [value] if !value.is_a?(Array)
      value.each do |v|
        hidden_fields << hidden_field_tag(name, v.to_s, :id => nil)          

Here is the output of my helpers for our example hash above:

  > puts CGI.unescape(url_for(flatten_hash(params)))
  > /contacts/search?person[personal][interests][][]=music&person[personal][interests][][]=chess

  > puts hash_as_hidden_fields(params)
  > <input name="action" type="hidden" value="search" />
  > <input name="controller" type="hidden" value="contacts" />
  > <input name="person[personal][interests][]" type="hidden" value="music" />
  > <input name="person[personal][interests][]" type="hidden" value="chess" />

Hope this will save other Rails programmers some time and frustration.