Attach ActiveStorage blob with a different filename

The constraints of Active Storage mean that you definitely will need to duplicate the blob. Within Active Storage, the blob key is indexed uniquely, so it cannot be referenced twice by different filenames.

If your storage service allows you to duplicate the blob efficiently in situ, then the enabling method on the Rails side is create_before_direct_upload!. Despite the somewhat misleading name, this method is specifically documented to provide a new (but unattached) blob record, in expectation that the content itself will be uploaded outside of the application. The blob record is then attached in a subsequent operation.

That is how Direct Upload works, and we can hook into the same two-phase mechanism; in this special case, the "upload" will instead be a service-specific duplication.

The means to duplicate in-situ vary according to the storage service, but for example:

• S3's CopyObject call,
• The AWS S3 Ruby SDK's copy_to method,
• A Unix file system's hard link.

An illustrative chunk of demonstration code therefore might be:

def blob_clone(current_blob, **options)
  blob_parameters = { 
    filename: current_blob.filename,
    byte_size: current_blob.byte_size,
    checksum: current_blob.checksum,
    content_type: current_blob.content_type,
    metadata: current_blob.metadata,
    **options
  }

  service = current_blob.service
  new_blob = ActiveStorage::Blob.create_before_direct_upload!(**blob_parameters)

  case service.class.name
  when 'ActiveStorage::Service::S3Service'
    bucket = service.bucket
    current_object = bucket.object(current_blob.key)
    current_object.copy_to(bucket.object(new_blob.key))
  when 'ActiveStorage::Service::DiskService'
    current_path = service.path_for(current_blob.key)
    new_path = service.path_for(new_blob.key)
    FileUtils.mkdir_p(File.dirname(new_path))
    FileUtils.ln(current_path, new_path)
  else
    raise ArgumentError, "unknown blob storage service #{service.class.name}"
  end

  new_blob
end

to be used thus:

current_blob = mymodel.contract.blob
new_blob = blob_clone(current_blob, filename: 'newfilename.pdf')
new_model.contract.attach(new_blob)

although I recommend refactoring this example according to your application structure and preferences.

Note on the code

In the illustration above, the case service.class.name is rather unlovely, and is only written so to keep the demo code self-contained. The problem being that Rails doesn't even load the class constant for an unused backend storage service. If you are not using diverse storage services across environments, then the case-statement may be unnecessary, and if this were fully realised as a gem, or refactored into an initializer, I'd probably finesse it to be monkey-patching/refining the service classes themselves.

If you want to point to the same blob but with a different name, maybe you can try to add the needed record in the database.

This is the schema for the active storage attachments table:

  create_table "active_storage_attachments", force: :cascade do |t|
    t.string "name", null: false
    t.string "record_type", null: false
    t.bigint "record_id", null: false
    t.bigint "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

You should be able to add a new record with almost the same values and a different name.

Maybe something like:

my_object.my_attachments.create(blob_id: my_object.document_blob.id, name: 'new name')

The StringIO can convert downloaded blob to IO then you can use any name you want.

attachements.push([
  io: StringIO.new( contract.document.download ),
  filename: "New contract filename"
])

ActiveStorage creates two tables active_storage_blobs which includes the uploaded files information like (filename) and active_storage_attachments which is a join table between a model record and a blob.

# rails console
=> ActiveStorage::Attachment
(id: integer, name: string, record_type: string, record_id: integer, blob_id: integer, created_at: datetime)
=> ActiveRecord::Blob
(id: integer, key: string, filename: string, content_type: string, metadata: text, byte_size: integer, checksum: string, created_at: datetime)

To change the filename of a blob supposing that

contract model has_many_attachments :documents

contract_object.documents.first.blob.update(filename: 'new file name')

To duplicate a blob you can pass a blob object to attach, then rename the filename