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Install

rails active_storage:install

ActiveStorage easily create 2 tables active_storage_blobs and active_storage_attachments

  • active_storage_blobs save about blob file's information such as: key, filename, content_type, meta_data, byte_size,...

  • active_storage_attachments is a join table between record and blobs, use polymorphic technique

Where to start?

rails g scaffold user name:string email: string
class User < ActiveRecord::Base
  has_one_attached :avatar
end

ActiveStorage will inject methods for associations between User model and ActiveStorage::Blob and ActiveStorage::Attachment as well as callbacks for deleting files when delete User.

Change User's form and controller:

# app/views/users/_form.html.erb

<div class="field">
  <%= form.label :avatar %>
  <%= form.file_field :avatar %>
</div>
# app/controllers/users_controller.rb
def user_params
  params.require(:user).permit(:email, :name, :avatar)
end

When submit the form, it will pass :avatar as ActionDispatch::Http::UploadedFile to User:

TODO image

And then, ActiveStorage will use switch case to choose right action to perform.

https://github.com/rails/rails/blob/master/activestorage/lib/active_storage/attached.rb#L18

How ActiveStorage save file to store service and save blob record?

def create_after_upload!(io:, filename:, content_type: nil, metadata: nil, identify: true)
  build_after_upload(io: io, filename: filename, content_type: content_type, metadata: metadata, identify: identify).tap(&:save!)
end

Why ActiveStorage build before save?

Avoid uploading (take a long time) while having an open database transaction

How ActiveStorage specify where to store file?

Inside build_after_upload, ActiveStorage will trigger upload to store service with a key https://github.com/rails/rails/blob/master/activestorage/app/models/active_storage/blob.rb#L154

key just a token key which generated by:

class ActiveStorage::Blob < ActiveRecord::Base
  has_secure_token :key
end

Disk

ActiveStorage use IO.copy_stream to upload file

  • Where?

    ActiveStorage uses key[0..1]/key[2..3] folder to store file.

S3

ActiveStorage just use aws-sdk-s3 to put file with key:

def upload(key, io, checksum: nil)
  instrument :upload, key: key, checksum: checksum do
    begin
      object_for(key).put(upload_options.merge(body: io, content_md5: checksum))
    rescue Aws::S3::Errors::BadDigest
      raise ActiveStorage::IntegrityError
    end
  end
end

How ActiveStorage generate url for a file when upload done?

ActiveStorage use store service to generate the URL for file

S3

def url(key, expires_in:, filename:, disposition:, content_type:)
  ...
    generated_url = object_for(key)...

    generated_url
  ...
end

S3 will handle expiration for us.

Disk is more complex

  • How to implements expires_in? When you request the image, it will send to ActiveStorage::DiskController with params[:encoded_key], and ActiveStorage use these key to find out the key of image. This key contains expires_in value, so only image which is fresh will be return or ActiveStorage::DiskController will return :not_found.

But what happen when show image http://localhost:3000/users/3, ActiveStorage will generate a new URL with new expires_in

https://github.com/rails/rails/blob/master/activestorage/app/controllers/active_storage/blobs_controller.rb#L12

For more information about how Rails encrypt the message with expires_in: https://medium.com/@michaeljcoyne/rails-5-2-metadata-options-for-messageverifier-and-messageencryptor-79540de86f9b

Variants

ActiveStorage supports image transform with ImageMagick, just add gem mini_magick to application's Gemfile.

Processed image will be saved into storage/va/ri/variants/:original_key/:Digest::SHA256.hexdigest(variation.key) https://github.com/rails/rails/blob/master/activestorage/app/models/active_storage/variant.rb#L71

upload and download are just like Blob

Preview