Converts a nested hash into a flat hash with bracketed keys as needed by http multipart form data

convert_params_to_form_data.rb

# Flatten a hash's keys down and format them like http form data
# e.g. { user: { id: 1, posts: [{ name: 'test' }] }, debug: true }
#      becomes
#      { 'user[id]' => 1, 'user[posts][0][name]' => 'test', 'debug' => true }
def convert_params_to_form_data(params, form_data = {}, prefix = '')
  bracketeer = ->(p, k) { "#{p}#{p.empty? ? '' : '['}#{k}#{p.empty? ? '' : ']'}" }

  params.each_with_object form_data do |(key, value), form_data|
    case value when Hash
      convert_params_to_form_data value, form_data, bracketeer.call(prefix, key)
    when Array
      value.each_with_index do |v, i|
        convert_params_to_form_data v, form_data, "#{prefix}[#{key}][#{i}]"
      end
    else
      form_data[bracketeer.call(prefix, key)] = value
    end
  end
end