Turn an arbitrary hash into hidden fields on a Rails form

hash_fields_helper.rb

module HashFieldsHelper
  def flatten_hash(hash, ancestor_names = [])
    hash.map do |k, v|
      case v
      when Hash
        flatten_hash(v, ancestor_names + [k])
      when Array
        v.map { |item| flatten_hash(item, ancestor_names + [k, '']) }.inject(:merge)
      else
        {flat_hash_key(ancestor_names + [k]) => v.to_s}
      end
    end.inject(:merge)
  end

  def flat_hash_key(names)
    names.first + names[1..-1].map { "[#{_1}]" }.join
  end

  def hash_as_hidden_fields(hash, ancestor_names = [])
    flatten_hash(hash, ancestor_names).map do |name, value|
      hidden_field_tag(name, value.to_s, id: nil)
    end.join("\n").html_safe
  end
end