retrieving_uploads.md

doc/retrieving_uploads.md
Last Update: 2018-12-27 17:29:27 +0100

Retrieving Uploads

Uploaded file content is typically retrieved from the storage using a Shrine::UploadedFile object. This guide explains the various methods of retrieving file content and how do they work.

For context, Shrine::UploadedFile object is what is returned by the attachment reader method on the model instance (e.g. photo.image), Shrine::Attacher#get if you're using the attacher directly, or Shrine#upload if you're using the uploader directly.

IO-like interface

In order for Shrine::UploadedFile objects to be uploadable to a storage, they too conform to Shrine's IO-like interface, meaning they implement #read, #rewind, #eof?, and #close matching the behaviour of the same methods on Ruby's IO class.

uploaded_file.eof?   # => false
uploaded_file.read   # => "..."
uploaded_file.eof?   # => true
uploaded_file.rewind # rewinds the underlying IO object
uploaded_file.eof?   # => false
uploaded_file.close  # closes the underlying IO object (this should be called when you're done)

In reality these methods are simply delegated on the IO object returned by the Storage#open method of the underlying Shrine storage. For Shrine::Storage::FileSystem this IO object will be a File object, while for Shrine::Storage::S3 (and most other remote storages) it will be a Down::ChunkedIO object. Storage#open is implicitly called when any of these IO methods are called for the first time.

uploaded_file.read(10) # calls `Storage#open` and assigns result to an instance variable
uploaded_file.read(10)
# ...

You can retrieve the underlying IO object returned by Storage#open with #to_io:

uploaded_file.to_io # the underlying IO object returned by `Storage#open`

Opening

The Shrine::UploadedFile#open method can be used to open the uploaded file explicitly:

uploaded_file.open # calls `Storage#open` and assigns result to an instance variable
uploaded_file.read
uploaded_file.close

This is useful if you want to control where Storage#open will be called. It's also useful if you want to pass additional parameters to Storage#open, which will depend on the storage. For example, if you're using S3 storage and server-side encryption, you can pass the necessary server-side-encryption parameters to Shrine::Storage::S3#open:

# server-side encryption parameters for S3 storage
uploaded_file.open(
 sse_customer_algorithm: "AES256",
 sse_customer_key:       "secret_key",
 sse_customer_key_md5:   "secret_key_md5",
)

Shrine::UploadedFile#open also accepts a block, which will ensure that the underlying IO object is closed at the end of the block.

uploaded_file.open do
  uploaded_file.read(1000)
  # ...
end # underlying IO object is closed

Shrine::UploadedFile#open will return the result of a given block. block. We can use that to safely retrieve the whole content of a file, without leaving any temporary files lying around.

content = uploaded_file.open(&:read) # open, read, and close
content # uploaded file content

Streaming

The Shrine::UploadedFile#stream method can be used to stream uploaded file content to a writable destination object.

destination = StringIO.new # from the "stringio" standard library
uploaded_file.stream(destination)
destination.rewind

destination # holds the file content

The destination object can be any object that responds to #write and returns number of bytes written, or a path string.

Shrine::UploadedFile#stream will play nicely with Shrine::UploadedFile#open, meaning it will not re-open the uploaded file if it's already opened.

uploaded_file.open do
  uploaded_file.stream(destination)
end

Any additional parameters to Shrine::UploadeFile#stream are forwarded to Storage#open. For example, if you're using S3 storage, you can tell AWS S3 to use HTTP compression for the download request:

uploaded_file.stream(destination, response_content_encoding: "gzip")

If you want to stream uploaded file content to the response body in a Rack application (Rails, Sinatra, Roda etc), see the rack_response plugin.

Downloading

The Shrine::UploadedFile#download method can be used to download uploaded file content do disk. Internally a temporary file will be created (using the tempfile standard library) and passed to Shrine::UploadedFile#stream. The return value is an open Tempfile object (a delegate of the File class).

tempfile = uploaded_file.download
tempfile #=> #<Tempfile:...>

tempfile.path   #=> "/var/folders/k7/6zx6dx6x7ys3rv3srh0nyfj00000gn/T/20181227-2915-m2l6c1"
tempfile.read   #=> "..."
tempfile.close! # close and unlink

Like Shrine::UploadedFile#open, Shrine::UploadedFile#download accepts a block as well. The Tempfile object is yielded to the block, and after the block finishes it's automatically closed and deleted.

uploaded_file.download do |tempfile|
  tempfile.path   #=> "/var/folders/k7/6zx6dx6x7ys3rv3srh0nyfj00000gn/T/20181227-2915-m2l6c1"
  tempfile.read   #=> "..."
  # ...
end # tempfile is closed and deleted

Since Shrine::UploadedFile#download internally uses Shrine::UploadedFile#stream, it plays nicely with Shrine::UploadedFile#open as well, meaning it will only open the uploaded file if it's not already opened.

uploaded_file.open do
  tempfile = uploaded_file.download
  # ...
end

Any options passed to Shrine::UploadedFile#download are forwarded to Storage#open (unless the uploaded file was already opened, in which case Storage#open was already called). For example, if you're using S3 storage, you can tell AWS S3 to use HTTP compression for the download request:

uploaded_file.download(response_content_encoding: "gzip")

Every time Shrine::UploadedFile#download is called, it will make a new copy of the uploaded file content. If you plan to retrieve uploaded file content multiple times for the same Shrine::UploadedFile instance, consider using the tempfile plugin.