Caching Dropbox URLs for paperclip-dropbox (not fully tested; not handling "missing" properly)

20130128194931_add_attachment_avatar_to_users.rb

class AddAttachmentAvatarToUsers < ActiveRecord::Migration
  def self.up
    change_table :users do |t|
      t.attachment :avatar
    end

    add_column :users, :avatar_dropbox, :string, :default => {}.to_yaml
  end

  def self.down
    drop_attached_file :users, :avatar
    remove_column :users, :avatar_dropbox
  end
end

application_helper.rb

  module ApplicationHelper
    def dropbox_attachment_url(object, attribute, style = :original)
      base = "#{attribute}_dropbox"
      cache = object.send(base) || {}
      cache[style] ||= {}
      cache[style][:last_request] ||= Time.now - 4.hours
  
      if Time.now - cache[style][:last_request] > 4.hours - 1.minute
        attachment = object.send(attribute, style)
  
        cache[style][:url] = attachment
        cache[style][:last_request] = Time.now
  
        object.update_attributes({base => cache})
  
        cache[style][:url]
      else
        object.send(base)[style][:url]
      end
    end
  end

user.rb

class User < ActiveRecord::Base
  serialize :avatar_dropbox
end

views\users\show.html.erb

<%= tag(:img, { :src => dropbox_attachment_url(@user, :avatar) }, false) %>