avatars_controller.rb

class AvatarsController < ApplicationController
  before_filter :new_file_upload, :only => :new

  def create
    user = User.find params[:user_id]
    user.avatar_key = params[:key]
    user.save!
    flash[:notice] = "Your new avatar is thumbnailing and will be ready soon."
    redirect_to user_url(user)
  end

  protected

  def new_file_upload
    expiration = 10.hours.from_now
    key = "users/#{current_user.to_param}/#{Time.now.strftime('%Y%m%d%H%M%S')}/avatar"

    @image_upload = Ungulate::FileUpload.new(
      :bucket_url => "http://#{S3_CONFIG['bucket']}.s3.amazonaws.com/",
      :key => key,
      :policy => {
        'expiration' => expiration,
        'conditions' => [
          {'bucket' => S3_CONFIG['bucket']},
          {'key' => key},
          {'acl' => 'private'},
          {'success_action_redirect' => create_user_avatar_url(current_user)}
        ]
      }
    )
  end
end

new.html.erb

<% ungulate_upload_form_for(@image_upload) do %>
  <%= label_tag :file, 'Avatar' %>
  <%= file_field_tag :file %>
  <%= submit_tag %>
<% end %>

user.rb

class User < ActiveRecord::Base

  after_update Proc.new {|user| user.ungulate_enqueue if user.avatar_key_changed?}

  def ungulate_enqueue
    Ungulate::FileUpload.enqueue(
      :bucket => S3_CONFIG['bucket'],
      :key => avatar_key,
      :versions => {
        :thumbnail => [ :resize_to_fill, 64, 64 ],
        :tiny => [ :resize_to_fill, 24, 24 ],
      }
    )
  end
end