Stubbing out Paperclip/ImageMagick in Tests

May 19, 2010 Pivotal Labs

Many people use the ultra popular Paperclip library to handle file attachments in Rails. Unfortunately the Paperclip documentation does not cover how to stub out calls to ImageMagick in your test suite. Without the proper stubs in place a test suite that uses Paperclip will take much, much longer to run.

In the grease your suite presentation by Nick Gauthier it has a slide titled Quickerclip that describes what needs to be done to spend up Paperclip in tests, basically you need to keep it from shelling out to ImageMagick. Alas, the presentation does include code for how to achieve Quickerclip.

As the presentation shows is the method that needs to be changed. The first parameter passed to is the ImageMagick command be executed. Paperclip uses the identify and convert commands. The identify command is used to determine the dimensions of an image. The convert command is the really heavy one that does image manipulation and thumbnail generation. Here is a redefinition of with sensible behavior for tests.

module Paperclip
  def cmd, params = "", expected_outcodes = 0
    case cmd
    when "identify"
      return "100x100"
    when "convert"

class Paperclip::Attachment
  def post_process

Redefining post_process in Paperclip::Attachment is an optional additional optimization. In Paperclip, post_process eventually calls"convert") and by short-circuiting the method earlier in the chain we save a few cycles.

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