paperclip.rb

# config/initializers/paperclip.rb
# If you want to disable paperclip attachments to S3 in the development environment,
# remove the comments from the "if" condition below and restart the server.
#if Rails.env.staging? || Rails.env.production?
  Paperclip::Attachment.default_options[:storage] = :s3

  Paperclip::Attachment.default_options[:s3_credentials] = {
      :bucket => "app-#{Rails.env}",
      :access_key_id => '<ACCESS_KEY_ID>',
      :secret_access_key => '<SECRET_ACCESS_KEY>'
  }

  Paperclip::Attachment.default_options[:url] = ':s3_domain_url'
  Paperclip::Attachment.default_options[:path] = '/:class/:attachment/:id_partition/:style/:filename'
  # The only addition required to make the code work with DreamHost's DreamObjects..
  Paperclip::Attachment.default_options[:s3_host_name] = 'objects.dreamhost.com'
  Paperclip::Attachment.default_options[:s3_protocol] = 'https'
#end

paperclip_migration.rake

# First setup the associated S3 config for paperclip (the attached initializer would do).
# Then add the classes your want to migrate in the klasses array below.
# Then run rake paperclip_migration:migrate_to_s3

namespace :paperclip_migration do  
  desc "migrate files from filesystem to s3"
  task :migrate_to_s3 => :environment do
    klasses = [:model_1, :model_2] # Replace with your real model names.

    klasses.each do |klass_key|
      klass = klass_key.to_s.classify.constantize rescue nil

      if klass.blank?
        puts "#{klass_key.to_s.classify} is not defined in this app."
        next
      end

      definitions = klass.respond_to?(:attachment_definitions) ? klass.attachment_definitions : nil

      if definitions.blank?
        puts "There are no paperclip attachments defined for the class #{klass.to_s}"
        next
      end

      definitions.keys.each do |attachment|
        records = klass.where("#{attachment}_file_name is not NULL")

        records.each do |record|
          path = record.send(attachment).path(:original)
          full_path = File.join( Rails.root, 'public', 'system', path )

          unless File.exists?( full_path )
            puts "Can't find file: #{full_path} NOT MIGRATING..."
            next
          end

          file_data = File.open( full_path )
          puts "Saving file: #{full_path} to Amazon S3..."
          record.send("#{attachment}=", file_data)
          record.save!
          file_data.close
        end
      end
    end
  end
end

thanks, you made my morning

Awesome, thanks for this. I think it is slightly out of date now that it's been over two years, it almost worked, but to get mine working with dream objects (latest version of paperclip, aws-sdk, and using rails 5) I had to set it up my initializer like this:

Paperclip::Attachment.default_options[:storage] = :s3

Paperclip::Attachment.default_options[:s3_credentials] = {
  :bucket => ENV['AWS_BUCKET'],
  :access_key_id => ENV['AWS_KEY'],
  :secret_access_key => ENV['AWS_SECRET_KEY'],
  s3_region: 'us-west-1'
}

Paperclip::Attachment.default_options[:s3_options] = {
  endpoint: 'https://objects-us-west-1.dream.io'
}

Paperclip::Attachment.default_options[:url] = ':s3_domain_url'
Paperclip::Attachment.default_options[:path] = '/:class/:attachment/:id_partition/:style/:filename'
Paperclip::Attachment.default_options[:s3_host_name] = 'objects-us-west-1.dream.io'
Paperclip::Attachment.default_options[:s3_protocol] = 'https'