/ Ruby on Rails
/
method

has_one_attached

has_one_attached(name, dependent: :purge_later, service: nil, strict_loading: false)
public

Specifies the relation between a single attachment and the model.

class User < ApplicationRecord
  has_one_attached :avatar
end

There is no column defined on the model side, Active Storage takes care of the mapping between your records and the attachment.

To avoid N+1 queries, you can include the attached blobs in your query like so:

User.with_attached_avatar

Under the covers, this relationship is implemented as a has_one association to a ActiveStorage::Attachment record and a has_one-through association to a ActiveStorage::Blob record. These associations are available as avatar_attachment and avatar_blob. But you shouldn’t need to work with these associations directly in most circumstances.

The system has been designed to having you go through the ActiveStorage::Attached::One proxy that provides the dynamic proxy to the associations and factory methods, like attach.

The :dependent option defaults to :purge_later. This means the attachment will be purged (i.e. destroyed) in the background whenever the record is destroyed. If an ActiveJob::Backend queue adapter is not set in the application set it to purge instead.

If you need the attachment to use a service which differs from the globally configured one, pass the :service option. For example:

class User < ActiveRecord::Base
  has_one_attached :avatar, service: :s3
end

:service can also be specified as a proc, and it will be called with the model instance:

class User < ActiveRecord::Base
  has_one_attached :avatar, service: ->(user) { user.in_europe_region? ? :s3_europe : :s3_usa }
end

If you need to enable strict_loading to prevent lazy loading of attachment, pass the :strict_loading option. You can do:

class User < ApplicationRecord
  has_one_attached :avatar, strict_loading: true
end

Note: Active Storage relies on polymorphic associations, which in turn store class names in the database. When renaming classes that use has_one_attached, make sure to also update the class names in the active_storage_attachments.record_type polymorphic type column of the corresponding rows.