has_attached_file :image, :styles => { ...... }

(record.image.styles.keys+[:original]).each do |style|
    path = record.image.path(style)
    FileUtils.move(path, File.join(File.dirname(path), new_file_name))
end

record.image_file_name = new_file_name
record.save

params[:upload][:md5] = Digest::MD5.file(file.path).hexdigest

Paperclip.interpolates :md5 do|attachment,style|
  attachment.instance.md5
end

validates_attachment_presence :upload
has_attached_file :upload,
  :path => ':rails_root/public/files/:md5.:extension',
  :url => '/files/:md5.:extension'

(record.image.styles.keys+[:original]).each do |style|
  AWS::S3::S3Object.move_to record.image.path(style), new_file_path, record.image.bucket_name
end

record.update_attribute(:image_file_name, new_file_name)

class User < ActiveRecord::Base
  after_update :rename_attached_files_if_needed

  has_attached_file :avatar_image,
    :storage        => :s3,
    :s3_credentials => "#{Rails.root}/config/s3.yml",
    :path           => "/users/:id/:style/:slug.:extension",
    :default_url    => "/images/users_default.gif",
    :styles         => { mini: "50x50>", normal: "100x100>", bigger: "150x150>" }

  def slug
    return name.parameterize if name
    "unknown"
  end


  def rename_attached_files_if_needed
    return if !name_changed? || avatar_image_updated_at_changed?
    (avatar_image.styles.keys+[:original]).each do |style|
      extension = Paperclip::Interpolations.extension(self.avatar_image, style)
      old_path = "users/#{id}/#{style}/#{name_was.parameterize}#{extension}"
      new_path = "users/#{id}/#{style}/#{name.parameterize}#{extension}"
      avatar_image.s3_bucket.objects[old_path].move_to new_path, acl: :public_read
    end
  end
end

def rename(key, new_name)
    file_name = (key.to_s+"_file_name").to_sym
    old_name = self.send(file_name)
    (self.send(key).styles.keys+[:original]).each do |style|
      path = self.send(key).path(style)
      self[file_name] = new_name
      new_path = self.send(key).path(style)
      new_path[0] = ""
      self[file_name] = old_name
      old_obj = self.send(key).s3_object(style.to_sym)
      new_obj = old_obj.move_to(new_path)
    end
    self.update_attribute(file_name, new_name)
  end

styles = file.styles.keys+[:original]
old_style2key = Hash[ styles.collect{|s| [s,file.path(s).sub(%r{A/},'')]} ]
self.file_file_name = new_filename
new_style2key = Hash[ styles.collect{|s| [s,file.path(s).sub(%r{A/},'')]} ]

styles.each do |style|
  raise "same key" if old_style2key[style] == new_style2key[style]
  file.s3_bucket.objects[old_style2key[style]].copy_to(new_style2key[style])
end

save!

styles.each do |style|
  file.s3_bucket.objects[old_style2key[style]].delete
end

Paperclip::Attachment.default_options.update({
    # :url=>"/system/:class/:attachment/:id_partition/:style/:filename"
    :url=>"/system/:class/:attachment/:style/:fingerprint.:extension"
    })

:path=>":rails_root/public:url"

$ rails c --sandbox
> Paperclip::Attachment.default_options
..
> s = User.create(:avatar => File.open('/foo/bar.jpg', 'rb'))
..
> s.avatar.path
 => "/home/groovy_user/rails_projectes/funky_app/public/system/users/avatars/original/49332b697a83d53d3f3b5bebce7548ea.jpg"
> s.avatar.url
 => "/system/users/avatars/original/49332b697a83d53d3f3b5bebce7548ea.jpg?1387099146"

class RenamePhotoColumnFromUsers < ActiveRecord::Migration
  def up
    add_attachment :users, :photo

    # Add `avatar` method (from Paperclip) temporarily, because it has been deleted from the model
    User.has_attached_file :avatar, styles: { medium: '300x300#', thumb: '100x100#' }
    User.validates_attachment_content_type :avatar, content_type: %r{Aimage/.*Z}

    # Copy `avatar` attachment to `photo` in S3, then delete `avatar`
    User.where.not(avatar_file_name: nil).each do |user|
      say "Updating #{user.email}..."

      user.update photo: user.avatar
      user.update avatar: nil
    end

    remove_attachment :users, :avatar
  end

  def down
    raise ActiveRecord::IrreversibleMigration
  end
end

def sanitize_filename(filename)
    fn = filename.split /(?<=.).(?=[^.])(?!.*.[^.])/m
    fn[0] = fn[0].parameterize
    return fn.join '.'
end

include ApplicationHelper

Paperclip::Attachment.default_options.update({
    :path => ":rails_root/public/system/:class/:attachment/:id/:style/:parameterize_file_name",
    :url => "/system/:class/:attachment/:id/:style/:parameterize_file_name",
})

Paperclip.interpolates :parameterize_file_name do |attachment, style|
    sanitize_filename(attachment.original_filename)
end

attr_accessor :avatar_update_file_name
  has_attached_file :avatar,
                    :styles => {
                      :medium => "288x288#",
                      :small => "150x150#",
                      :thumb => "48x48#"
                    },
                    :path => YOUR_PATH + ":style/:basename.:extension",
                    :url => YOUR_PATH + ":style/:filename",
                    preserve_files: true


  process_in_background :avatar, only_process: [:medium, :small, :thumb, :original]
  after_save :rename_avatars

  def rename_avatars
    if self.avatar_update_file_name == true
      new_file_hash = "#{SecureRandom.urlsafe_base64}"
      new_file_name = ""
      (self.avatar.styles.keys+[:original]).each do |style|
        path = avatar.path(style)
        if(File.file?(path))
        File.chmod(0777, File.dirname(path))
        new_file_name = "#{new_file_hash}.#{extension}"
        FileUtils.move(path, File.join(File.dirname(path), new_file_name))
        end
      end
      self.avatar_update_file_name = false
      self.avatar_file_name = new_file_name
      self.save!
    end
  end

avatar = new Avatars
#... upload and work with files
avatar.avatar_update_file_name = true
avatar.save!