policy.rb

class Policy
  attr_reader :expires, :response

  def initialize user
    if user
      @expires = 8.hours.from_now
      content_length = 100..50.megabytes
      user_id = user.email.split('@').reverse.join('/')
    else
      @expires = 30.minutes.from_now
      content_length = 100..15.megabytes
      user_id = "anonymous"
    end

    s3 = AWS::S3.new
    bucket = s3.buckets['api.octanner.com-temp-upload']

    # Build a signed S3 policy & upload form
    upload = bucket.presigned_post(expires: @expires, content_length: content_length)
      .where(:key).starts_with("upload/#{user_id}/")
      .where(:acl).is("public-read")
      .where(:content_type).starts_with("image/")
      .where(:success_action_status).is("201")
      .where_metadata("user-id").is(user_id)

    # Create a JSON response explaining how to build the <form>
    @response = {
      action: upload.url.to_s,
      method: 'POST',
      type: 'multipart/form-data',
      input: {
        acl: 'public-read',
        'Content-Type' => 'image/jpeg',
        'x-amz-meta-user-id' => user_id,
        'success_action_status' => '201',
        key: { type: 'string', prefix: "upload/#{user_id}/" }
      }
    }

    # Bring in the fields created by the policy builder
    upload.fields.each do |name, value|
      @response[:input][name] = value unless value.nil?
    end

    # And require a file
    @response[:input][:file] = { type: 'file' }
  end
end