s3_uploader.rb

class S3Uploader
  attr_reader :expiration, :path

  def initialize
    @path = "uploads/#{SecureRandom.hex}"
    @expiration = 10.hours.from_now
  end

  def access_key_id
    ENV['AWS_ACCESS_KEY_ID']
  end

  def secret_access_key
    ENV['AWS_SECRET_ACCESS_KEY']
  end

  def bucket
    ENV['AWS_S3_BUCKET']
  end

  def key
    "#{path}/${filename}"
  end

  def acl
    'public-read'
  end

  def max_file_size
    500.megabytes
  end

  def fields
    {
      :key => key,
      :acl => acl,
      :policy => policy,
      :signature => signature,
      "AWSAccessKeyId" => access_key_id,
    }
  end

  def url
    "https://#{bucket}.s3.amazonaws.com/"
  end

  def policy
    Base64.encode64(policy_data.to_json).gsub("\n", "")
  end

  def policy_data
    {
      expiration: expiration,
      conditions: [
        ["starts-with", "$utf8", ""],
        ["starts-with", "$key", "uploads/"],
        ["content-length-range", 0, max_file_size],
        {bucket: bucket},
        {acl: acl}
      ]
    }
  end

  def signature
    Base64.encode64(
      OpenSSL::HMAC.digest(
        OpenSSL::Digest::Digest.new('sha1'),
        secret_access_key, policy
      )
    ).gsub("\n", "")
  end
end

upload_helper.rb

require 's3uploader'

module UploadHelper
  def photo_form_tag(options={}, &block)
    # Introduce standard form options.
    options = {
      method: "post",
      authenticity_token: false,
      multipart: true
    }.deep_merge(options)

    form = ImageUploader.file? ? :rails_uploader_form : :s3_uploader_form
    send(form, options, &block)
  end

  def rails_uploader_form(options={}, &block)
    form_tag(upload_photos_path, {multipart: true}.merge(options), &block)
  end

  def s3_uploader_form(options={}, &block)
    uploader = S3Uploader.new

    # The path the the file is uploaded to, not used by rails uploader.
    options.deep_merge!({
      data: {
        :'upload-path' => uploader.path
      }
    })

    form_tag(uploader.url, options) do
      uploader.fields.map do |name, value|
        hidden_field_tag(name, value)
      end.join.html_safe + capture(&block)
    end
  end
end