Active Storage Overview

1 What is Active Storage?

Active Storage facilitates uploading files to a cloud storage service like Amazon S3, Google Cloud Storage, or Microsoft Azure Storage and attaching those files to Active Record objects. It comes with a local disk-based service for development and testing and supports mirroring files to subordinate services for backups and migrations.

Using Active Storage, an application can transform image uploads with ImageMagick, generate image representations of non-image uploads like PDFs and videos, and extract metadata from arbitrary files.

2 Setup

Active Storage uses two tables in your application’s database named active_storage_blobs and active_storage_attachments. After upgrading your application to Rails 5.2, run rails active_storage:install to generate a migration that creates these tables. Use rails db:migrate to run the migration.

Declare Active Storage services in config/storage.yml. For each service your application uses, provide a name and the requisite configuration. The example below declares three services named local, test, and amazon:

local:
  service: Disk
  root: <%= Rails.root.join("storage") %>

test:
  service: Disk
  root: <%= Rails.root.join("tmp/storage") %>

amazon:
  service: S3
  access_key_id: ""
  secret_access_key: ""

Tell Active Storage which service to use by setting Rails.application.config.active_storage.service. Because each environment will likely use a different service, it is recommended to do this on a per-environment basis. To use the disk service from the previous example in the development environment, you would add the following to config/environments/development.rb:

# Store files locally.
config.active_storage.service = :local

To use the Amazon S3 service in production, you add the following to config/environments/production.rb:

# Store files on Amazon S3.
config.active_storage.service = :amazon

Continue reading for more information on the built-in service adapters (e.g. Disk and S3) and the configuration they require.

2.1 Disk Service

Declare a Disk service in config/storage.yml:

local:
  service: Disk
  root: <%= Rails.root.join("storage") %>

2.2 Amazon S3 Service

Declare an S3 service in config/storage.yml:

amazon:
  service: S3
  access_key_id: ""
  secret_access_key: ""
  region: ""
  bucket: ""

Add the aws-sdk-s3 gem to your Gemfile:

gem "aws-sdk-s3", require: false

The core features of Active Storage require the following permissions: s3:ListBucket, s3:PutObject, s3:GetObject, and s3:DeleteObject. If you have additional upload options configured such as setting ACLs then additional permissions may be required.

If you want to use environment variables, standard SDK configuration files, profiles, IAM instance profiles or task roles, you can omit the access_key_id, secret_access_key, and region keys in the example above. The Amazon S3 Service supports all of the authentication options described in the AWS SDK documentation.

2.3 Microsoft Azure Storage Service

Declare an Azure Storage service in config/storage.yml:

azure:
  service: AzureStorage
  storage_account_name: ""
  storage_access_key: ""
  container: ""

Add the azure-storage gem to your Gemfile:

gem "azure-storage", require: false

2.4 Google Cloud Storage Service

Declare a Google Cloud Storage service in config/storage.yml:

google:
  service: GCS
  credentials: <%= Rails.root.join("path/to/keyfile.json") %>
  project: ""
  bucket: ""

Optionally provide a Hash of credentials instead of a keyfile path:

google:
  service: GCS
  credentials:
    type: "service_account"
    project_id: ""
    private_key_id: <%= Rails.application.credentials.dig(:gcs, :private_key_id) %>
    private_key: <%= Rails.application.credentials.dig(:gcs, :private_key) %>
    client_email: ""
    client_id: ""
    auth_uri: "https://accounts.google.com/o/oauth2/auth"
    token_uri: "https://accounts.google.com/o/oauth2/token"
    auth_provider_x509_cert_url: "https://www.googleapis.com/oauth2/v1/certs"
    client_x509_cert_url: ""
  project: ""
  bucket: ""

Add the google-cloud-storage gem to your Gemfile:

gem "google-cloud-storage", "~> 1.8", require: false

2.5 Mirror Service

You can keep multiple services in sync by defining a mirror service. When a file is uploaded or deleted, it's done across all the mirrored services. Mirrored services can be used to facilitate a migration between services in production. You can start mirroring to the new service, copy existing files from the old service to the new, then go all-in on the new service. Define each of the services you'd like to use as described above and reference them from a mirrored service.

s3_west_coast:
  service: S3
  access_key_id: ""
  secret_access_key: ""
  region: ""
  bucket: ""

s3_east_coast:
  service: S3
  access_key_id: ""
  secret_access_key: ""
  region: ""
  bucket: ""

production:
  service: Mirror
  primary: s3_east_coast
  mirrors:
    - s3_west_coast

Files are served from the primary service.

3 Attaching Files to Records

3.1 `has_one_attached`

The has_one_attached macro sets up a one-to-one mapping between records and files. Each record can have one file attached to it.

For example, suppose your application has a User model. If you want each user to have an avatar, define the User model like this:

class User < ApplicationRecord
  has_one_attached :avatar
end

You can create a user with an avatar:

class SignupController < ApplicationController
  def create
    user = User.create!(user_params)
    session[:user_id] = user.id
    redirect_to root_path
  end

  private
    def user_params
      params.require(:user).permit(:email_address, :password, :avatar)
    end
end

Call avatar.attach to attach an avatar to an existing user:

Current.user.avatar.attach(params[:avatar])

Call avatar.attached? to determine whether a particular user has an avatar:

Current.user.avatar.attached?

3.2 `has_many_attached`

The has_many_attached macro sets up a one-to-many relationship between records and files. Each record can have many files attached to it.

For example, suppose your application has a Message model. If you want each message to have many images, define the Message model like this:

class Message < ApplicationRecord
  has_many_attached :images
end

You can create a message with images:

class MessagesController < ApplicationController
  def create
    message = Message.create!(message_params)
    redirect_to message
  end

  private
    def message_params
      params.require(:message).permit(:title, :content, images: [])
    end
end

Call images.attach to add new images to an existing message:

@message.images.attach(params[:images])

Call images.attached? to determine whether a particular message has any images:

@message.images.attached?

4 Removing Files

To remove an attachment from a model, call purge on the attachment. Removal can be done in the background if your application is setup to use Active Job. Purging deletes the blob and the file from the storage service.

# Synchronously destroy the avatar and actual resource files.
user.avatar.purge

# Destroy the associated models and actual resource files async, via Active Job.
user.avatar.purge_later

5 Linking to Files

Generate a permanent URL for the blob that points to the application. Upon access, a redirect to the actual service endpoint is returned. This indirection decouples the public URL from the actual one, and allows, for example, mirroring attachments in different services for high-availability. The redirection has an HTTP expiration of 5 min.

url_for(user.avatar)

To create a download link, use the rails_blob_{path|url} helper. Using this helper allows you to set the disposition.

rails_blob_path(user.avatar, disposition: "attachment")

6 Transforming Images

To create variation of the image, call variant on the Blob. You can pass any MiniMagick supported transformation to the method.

To enable variants, add mini_magick to your Gemfile:

gem 'mini_magick'

When the browser hits the variant URL, Active Storage will lazy transform the original blob into the format you specified and redirect to its new service location.

<%= image_tag user.avatar.variant(resize: "100x100") %>

7 Previewing Files

Some non-image files can be previewed: that is, they can be presented as images. For example, a video file can be previewed by extracting its first frame. Out of the box, Active Storage supports previewing videos and PDF documents.

<ul>
  <% @message.files.each do |file| %>
    <li>
      <%= image_tag file.preview(resize: "100x100>") %>
    </li>
  <% end %>
</ul>

Extracting previews requires third-party applications, ffmpeg for video and mutool for PDFs. These libraries are not provided by Rails. You must install them yourself to use the built-in previewers. Before you install and use third-party software, make sure you understand the licensing implications of doing so.

8 Direct Uploads

Active Storage, with its included JavaScript library, supports uploading directly from the client to the cloud.

8.1 Direct upload installation

Include activestorage.js in your application's JavaScript bundle.

Using the asset pipeline:
```
//= require activestorage
```
Using the npm package:
```
import * as ActiveStorage from "activestorage"
ActiveStorage.start()
```

Annotate file inputs with the direct upload URL.

<%= form.file_field :attachments, multiple: true, direct_upload: true %>

That's it! Uploads begin upon form submission.

8.2 Direct upload JavaScript events

Event name	Event target	Event data (`event.detail`)	Description
`direct-uploads:start`	`<form>`	None	A form containing files for direct upload fields was submitted.
`direct-upload:initialize`	`<input>`	`{id, file}`	Dispatched for every file after form submission.
`direct-upload:start`	`<input>`	`{id, file}`	A direct upload is starting.
`direct-upload:before-blob-request`	`<input>`	`{id, file, xhr}`	Before making a request to your application for direct upload metadata.
`direct-upload:before-storage-request`	`<input>`	`{id, file, xhr}`	Before making a request to store a file.
`direct-upload:progress`	`<input>`	`{id, file, progress}`	As requests to store files progress.
`direct-upload:error`	`<input>`	`{id, file, error}`	An error occurred. An `alert` will display unless this event is canceled.
`direct-upload:end`	`<input>`	`{id, file}`	A direct upload has ended.
`direct-uploads:end`	`<form>`	None	All direct uploads have ended.

8.3 Example

You can use these events to show the progress of an upload.

direct-uploads

To show the uploaded files in a form:

// direct_uploads.js

addEventListener("direct-upload:initialize", event => {
  const { target, detail } = event
  const { id, file } = detail
  target.insertAdjacentHTML("beforebegin", `
    <div id="direct-upload-${id}" class="direct-upload direct-upload--pending">
      <div id="direct-upload-progress-${id}" class="direct-upload__progress" style="width: 0%"></div>
      <span class="direct-upload__filename">${file.name}</span>
    </div>
  `)
})

addEventListener("direct-upload:start", event => {
  const { id } = event.detail
  const element = document.getElementById(`direct-upload-${id}`)
  element.classList.remove("direct-upload--pending")
})

addEventListener("direct-upload:progress", event => {
  const { id, progress } = event.detail
  const progressElement = document.getElementById(`direct-upload-progress-${id}`)
  progressElement.style.width = `${progress}%`
})

addEventListener("direct-upload:error", event => {
  event.preventDefault()
  const { id, error } = event.detail
  const element = document.getElementById(`direct-upload-${id}`)
  element.classList.add("direct-upload--error")
  element.setAttribute("title", error)
})

addEventListener("direct-upload:end", event => {
  const { id } = event.detail
  const element = document.getElementById(`direct-upload-${id}`)
  element.classList.add("direct-upload--complete")
})

Add styles:

/* direct_uploads.css */

.direct-upload {
  display: inline-block;
  position: relative;
  padding: 2px 4px;
  margin: 0 3px 3px 0;
  border: 1px solid rgba(0, 0, 0, 0.3);
  border-radius: 3px;
  font-size: 11px;
  line-height: 13px;
}

.direct-upload--pending {
  opacity: 0.6;
}

.direct-upload__progress {
  position: absolute;
  top: 0;
  left: 0;
  bottom: 0;
  opacity: 0.2;
  background: #0076ff;
  transition: width 120ms ease-out, opacity 60ms 60ms ease-in;
  transform: translate3d(0, 0, 0);
}

.direct-upload--complete .direct-upload__progress {
  opacity: 0.4;
}

.direct-upload--error {
  border-color: red;
}

input[type=file][data-direct-upload-url][disabled] {
  display: none;
}

9 Discarding Files Stored During System Tests

System tests clean up test data by rolling back a transaction. Because destroy is never called on an object, the attached files are never cleaned up. If you want to clear the files, you can do it in an after_teardown callback. Doing it here ensures that all connections created during the test are complete and you won't receive an error from Active Storage saying it can't find a file.

class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
  driven_by :selenium, using: :chrome, screen_size: [1400, 1400]

  def remove_uploaded_files
    FileUtils.rm_rf("#{Rails.root}/storage_test")
  end

  def after_teardown
    super
    remove_uploaded_files
  end
end

If your system tests verify the deletion of a model with attachments and you're using Active Job, set your test environment to use the inline queue adapter so the purge job is executed immediately rather at an unknown time in the future.

You may also want to use a separate service definition for the test environment so your tests don't delete the files you create during development.

# Use inline job processing to make things happen immediately
config.active_job.queue_adapter = :inline

# Separate file storage in the test environment
config.active_storage.service = :local_test

10 Implementing Support for Other Cloud Services

If you need to support a cloud service other than these, you will need to implement the Service. Each service extends ActiveStorage::Service by implementing the methods necessary to upload and download files to the cloud.

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