MIGRATING.md

Migrating from Paperclip to ActiveStorage

Paperclip and ActiveStorage solve similar problems with similar solutions, so transitioning from one to the other is straightforward data re-writing.

The process of going from Paperclip to ActiveStorage is as follows:

1. Apply the ActiveStorage database migrations.
2. Configure storage.
3. Copy the database data over.
4. Copy the files over.
5. Update your tests.
6. Update your views.
7. Update your controllers.
8. Update your models.

Apply the ActiveStorage database migrations

Follow the instructions for installing ActiveStorage. You'll very likely want to add the mini_magick gem to your Gemfile.

rails active_storage:install

Configure storage

Again, follow the instructions for configuring ActiveStorage.

Copy the database data over

The active_storage_blobs and active_storage_attachments tables are where ActiveStorage expects to find file metadata. Paperclip stores the file metadata directly on the associated object's table.

You'll need to write a migration for this conversion. Because the models for your domain are involved, it's tricky to supply a simple script. But we'll try!

Here's how it would go for a User with an avatar, that is this in Paperclip:

class User < ApplicationRecord
  has_attached_file :avatar
end

Your Paperclip migrations will produce a table like so:

create_table "users", force: :cascade do |t|
  t.string "avatar_file_name"
  t.string "avatar_content_type"
  t.integer "avatar_file_size"
  t.datetime "avatar_updated_at"
end

And you'll be converting into these tables:

create_table "active_storage_attachments", force: :cascade do |t|
  t.string "name", null: false
  t.string "record_type", null: false
  t.integer "record_id", null: false
  t.integer "blob_id", null: false
  t.datetime "created_at", null: false
  t.index ["blob_id"], name: "index_active_storage_attachments_on_blob_id"
  t.index ["record_type", "record_id", "name", "blob_id"], name: "index_active_storage_attachments_uniqueness", unique: true
end

create_table "active_storage_blobs", force: :cascade do |t|
  t.string "key", null: false
  t.string "filename", null: false
  t.string "content_type"
  t.text "metadata"
  t.bigint "byte_size", null: false
  t.string "checksum", null: false
  t.datetime "created_at", null: false
  t.index ["key"], name: "index_active_storage_blobs_on_key", unique: true
end

So, assuming you want to leave the files in the exact same place, this is your migration. Otherwise, see the next section first and modify the migration to taste.

Dir[Rails.root.join("app/models/**/*.rb")].sort.each { |file| require file }

class ConvertToActiveStorage < ActiveRecord::Migration[5.2]
  require 'open-uri'

  def up
    # postgres
    get_blob_id = 'LASTVAL()'
    # mariadb
    # get_blob_id = 'LAST_INSERT_ID()'
    # sqlite
    # get_blob_id = 'LAST_INSERT_ROWID()'

    active_storage_blob_statement = ActiveRecord::Base.connection.raw_connection.prepare(<<-SQL)
      INSERT INTO active_storage_blobs (
        key, filename, content_type, metadata, byte_size,
        checksum, created_at
      ) VALUES (?, ?, ?, '{}', ?, ?, ?)
    SQL

    active_storage_attachment_statement = ActiveRecord::Base.connection.raw_connection.prepare(<<-SQL)
      INSERT INTO active_storage_attachments (
        name, record_type, record_id, blob_id, created_at
      ) VALUES (?, ?, ?, #{get_blob_id}, ?)
    SQL

    models = ActiveRecord::Base.descendants.reject(&:abstract_class?)

    transaction do
      models.each do |model|
        attachments = model.column_names.map do |c|
          if c =~ /(.+)_file_name$/
            $1
          end
        end.compact

        model.find_each.each do |instance|
          attachments.each do |attachment|
            active_storage_blob_statement.execute(
              key(instance, attachment),
              instance.send("#{attachment}_file_name"),
              instance.send("#{attachment}_content_type"),
              instance.send("#{attachment}_file_size"),
              checksum(instance.send(attachment)),
              instance.updated_at.iso8601
            )

            active_storage_attachment_statement.
              execute(attachment, model.name, instance.id, instance.updated_at.iso8601)
          end
        end
      end
    end

    active_storage_attachment_statement.close
    active_storage_blob_statement.close
  end

  def down
    raise ActiveRecord::IrreversibleMigration
  end

  private

  def key(instance, attachment)
    SecureRandom.uuid
    # Alternatively:
    # instance.send("#{attachment}_file_name")
  end

  def checksum(attachment)
    # local files stored on disk:
    url = attachment.path
    Digest::MD5.base64digest(File.read(url))

    # remote files stored on another person's computer:
    # url = attachment.url
    # Digest::MD5.base64digest(Net::HTTP.get(URI(url)))
  end
end

Copy the files over

The above migration leaves the files as they are. However, the default Paperclip and ActiveStorage storage services use different locations.

By default, Paperclip looks like this:

public/system/users/avatars/000/000/004/original/the-mystery-of-life.png

And ActiveStorage looks like this:

storage/xM/RX/xMRXuT6nqpoiConJFQJFt6c9

That xMRXuT6nqpoiConJFQJFt6c9 is the active_storage_blobs.key value. In the migration above we simply used the filename but you may wish to use a UUID instead.

Migrating the files on an external file host (S3, Azure Storage, GCS, etc.) is out of scope for this initial document. Here is how it would look for local disk storage:

#!bin/rails runner

class ActiveStorageBlob < ActiveRecord::Base
end

class ActiveStorageAttachment < ActiveRecord::Base
  belongs_to :blob, class_name: 'ActiveStorageBlob'
  belongs_to :record, polymorphic: true
end

ActiveStorageAttachment.find_each do |attachment|
  name = attachment.name

  source = attachment.record.send(name).path
  dest_dir = File.join(
    "storage",
    attachment.blob.key.first(2),
    attachment.blob.key.first(4).last(2))
  dest = File.join(dest_dir, attachment.blob.key)

  FileUtils.mkdir_p(dest_dir)
  puts "Moving #{source} to #{dest}"
  FileUtils.cp(source, dest)
end

Update your tests

Instead of the have_attached_file matcher, you'll need to write your own. Here's one that is similar in spirit to the Paperclip-supplied matcher:

RSpec::Matchers.define :have_attached_file do |name|
  matches do |record|
    file = record.send(name)
    file.respond_to?(:variant) && file.respond_to?(:attach)
  end
end

Update your views

In Paperclip it looks like this:

image_tag @user.avatar.url(:medium)

In ActiveStorage it looks like this:

image_tag @user.avatar.variant(resize: "250x250")

Update your controllers

This should require no update. However, if you glance back at the database schema above, you may notice a join.

For example, if your controller has

def index
  @users = User.all.order(:name)
end

And your view has

<ul>
  <% @users.each do |user| %>
    <li><%= image_tag user.avatar.variant(resize: "10x10"), alt: user.name %></li>
  <% end %>
</ul>

Then you'll end up with an n+1 as you load each attachment in the loop.

So while the controller and model will work without change, you will want to double-check your loops and add includes as needed. ActiveStorage adds an avatar_attachment and avatar_blob relationship to has-one relations, and avatar_attachments and avatar_blobs to has-many:

def index
  @users = User.all.order(:name).includes(:avatar_attachment)
end

Update your models

Follow the guide on attaching files to records. For example, a User with an avatar is represented as:

class User < ApplicationRecord
  has_one_attached :avatar
end

Any resizing is done in the view as a variant.

Remove Paperclip

Remove the Gem from your Gemfile and run bundle. Run your tests because you're done!