Last Update: 2019-02-15 01:32:24 +0100

Testing with Shrine

The goal of this guide is to provide some useful tips for testing file attachments implemented with Shrine in your application.


When you first try to test file attachments, you might experience that files are simply not being promoted (uploaded from temporary to permanent storage). This is because your tests are likely setup to be wrapped inside database transactions, and that doesn't work with Shrine callbacks.

Specifically, Shrine uses “after commit” callbacks for promoting and deleting attached files. This means that if your tests are wrapped inside transactions, those Shrine actions will happen only after those transactions commit, which happens only after the test has already finished.

# Promoting will happen only after the test transaction commits
it "can attach images" do
  photo = Photo.create(image: image_file)
  photo.image.storage_key #=> :cache (we expected it to be promoted to permanent storage)

For file attachments to properly work, you'll need to disable transactions for those tests. For Rails apps you can tell Rails not to use transactions, and instead use libraries like DatabaseCleaner which allow you to use table truncation or deletion strategies instead of transactions.

RSpec.configure do |config|
  config.use_transactional_fixtures = false


If you're using FileSystem storage and your tests run in a single process, you can switch to memory storage, which is both faster and doesn't require you to clean up anything between tests.

# Gemfile
gem "shrine-memory"
# test/test_helper.rb
require "shrine/storage/memory"

Shrine.storages = {
  cache: Shrine::Storage::Memory.new,
  store: Shrine::Storage::Memory.new,

If you're using AWS S3 storage, you can use Minio (explained below) instead of S3, both in test and development environment. Alternatively, you can stub aws-sdk-s3 requests in tests.


Minio is an open source object storage server with AWS S3 compatible API which you can run locally. The advantage of using Minio for your development and test environments is that all AWS S3 functionality should still continue to work, including direct uploads, so you don't need to update your code.

If you're on a Mac you can install it with Homebrew:

$ brew install minio/stable/minio

Afterwards you can start the Minio server and give it a directory where it will store the data:

$ minio server data/

This command will print out the credentials for the running Minio server, as well as a link to the Minio web interface. Follow that link and create a new bucket. Once you've done that, you can configure Shrine::Storage::S3 to use your Minio server:

  access_key_id:     "<MINIO_ACCESS_KEY>", # "AccessKey" value
  secret_access_key: "<MINIO_SECRET_KEY>", # "SecretKey" value
  endpoint:          "<MINIO_ENDPOINT>",   # "Endpoint"  value
  bucket:            "<MINIO_BUCKET>",     # name of the bucket you created
  region:            "us-east-1",
  force_path_style:  true,

The :endpoint option will make aws-sdk-s3 point all URLs to your Minio server (instead of s3.amazonaws.com), and :force_path_style tells it not to use subdomains when generating URLs.

Test data

If you're creating test data dynamically using libraries like factory_bot, you can have the test file assigned dynamically when the record is created:

factory :photo do
  image { File.open("test/files/image.jpg") }

On the other hand, if you're setting up test data using Rails' YAML fixtures, you unfortunately won't be able to use them for assigning files. This is because Rails fixtures only allow assigning primitive data types, and don't allow you to specify Shrine attributes - you can only assign to columns directly.

Background jobs

If you're using background jobs with Shrine, you probably want to make them synchronous in tests. Your favourite backgrounding library should already support this, examples:

# Sidekiq
require "sidekiq/testing"
# SuckerPunch
require "sucker_punch/testing/inline"
# ActiveJob
ActiveJob::Base.queue_adapter = :inline

Acceptance tests

In acceptance tests you're testing your app end-to-end, and you likely want to also test file attachments here. There are a variety of libraries that you might be using for your acceptance tests.


If you're testing with the Capybara acceptance test framework, you can use #attach_file to select a file from your filesystem in the form:

attach_file("#image-field", "test/files/image.jpg")


Regular routing tests in Rails use Rack::Test, in which case you can create Rack::Test::UploadedFile objects and pass them as form parameters:

post "/photos", photo: {image: Rack::Test::UploadedFile.new("test/files/image.jpg", "image/jpeg")}


With Rack::TestApp you can create multipart file upload requests by using the :multipart option and passing a File object:

http.post "/photos", multipart: {"photo[image]" => File.open("test/files/image.jpg")}


Even though all the file attachment logic is usually encapsulated in your uploader classes, in general it's still best to test this logic through models.

In your controller the attachment attribute using the uploaded file from the controller, in Rails case it's an ActionDispatch::Http::UploadedFile. However, you can also assign plain File objects, or any other kind of IO-like objects.

describe ImageUploader do
  it "generates image thumbnails" do
    photo = Photo.create(image: File.open("test/files/image.png"))
    assert_equal [:small, :medium, :large], photo.image.keys

If you want test with an IO object that closely resembles the kind of IO that is assigned by your web framework, you can use this:

require "forwardable"
require "stringio"

class FakeIO
  attr_reader :original_filename, :content_type

  def initialize(content, filename: nil, content_type: nil)
    @io = StringIO.new(content)
    @original_filename = filename
    @content_type = content_type

  extend Forwardable
  delegate %i[read rewind eof? close size] => :@io
describe ImageUploader do
  it "generates image thumbnails" do
    photo = Photo.create(image: FakeIO.new(File.read("test/files/image.png")))
    assert_equal [:small, :medium, :large], photo.image.keys


In tests you usually don't want to perform processing, or at least don't want it to be performed by default (only when you're actually testing it).

If you're processing only single files, you can override the Shrine#process method in tests to return nil:

class ImageUploader
  def process(io, context)
    # don't do any processing

If you're processing versions, you can override Shrine#process to simply return a hash of unprocessed original files:

class ImageUploader
  def process(io, context)
    if context[:action] == :store
      {small: io.download, medium: io.download, large: io.download}

However, it's even better to design your processing code in such a way that it's easier to swap out in tests. In your application code you could extract processing into a single #call-able object, and register it inside uploader generic opts hash.

class ImageUploader < Shrine
  opts[:processor] = ImageThumbnailsGenerator

  process(:store) do |io, context|
    opts[:processor].call(io, context)

Now in your tests you can easily swap out ImageThumbnailsGenerator with “fake” processing, which just returns the result in correct format (single file or hash of versions). Since the only requirement of the processor is that it responds to #call, we can just swap it out for a proc or a lambda:

ImageUploader.opts[:processor] = proc do |io, context|
  # return unprocessed file(s)

This also has the benefit of allowing you to test ImageThumbnailsGenerator in isolation.