class ActiveStorage::Attached::One

person.avatar.attach(avatar_blob) # ActiveStorage::Blob object
person.avatar.attach(io: File.open("/path/to/face.jpg"), filename: "face.jpg", content_type: "image/jpeg")
person.avatar.attach(params[:signed_blob_id]) # Signed reference to blob from direct upload
person.avatar.attach(params[:avatar]) # ActionDispatch::Http::UploadedFile object

record is next saved.
the database immediately. Otherwise, it'll be saved to the DB when the
If the record is persisted and unchanged, the attachment is saved to

Attaches an +attachable+ to the record.

def attach(attachable)
  if record.persisted? && !record.changed?
    record.public_send("#{name}=", attachable)
    record.save
  else
    record.public_send("#{name}=", attachable)
  end
end

User.new.avatar.attached? # => false

end
has_one_attached :avatar
class User < ApplicationRecord

Returns +true+ if an attachment has been made.

def attached?
  attachment.present?
end

they are all available at the model level.
You don't have to call this method to access the attachment's methods as

Returns the associated attachment record.

def attachment
  change.present? ? change.attachment : record.public_send("#{name}_attachment")
end

User.new.avatar.blank? # => true

end
has_one_attached :avatar
class User < ApplicationRecord

Returns +true+ if an attachment is not attached.

def blank?
  !attached?
end

def detach_one
  Attached::Changes::DetachOne.new(name, record, attachment)
end

def purge_one
  Attached::Changes::PurgeOne.new(name, record, attachment)
end