I inherited a Rails 2.2.2 app that stores user-uploaded images on Amazon S3. The attachment_fu-based Photo model offers a rotate method that uses open-uri to retrieve the image from S3 and MiniMagick to perform the rotation.
The rotate method contains this line to retrieve the image for use with MiniMagick:
temp_image = MiniMagick::Image.from_file(open(self.public_filename).path)
self.public_filename returns something like
Retrieving the image and rotating it works just fine in the running application in production and development. However, the unit test fails with
TypeError: can't convert nil into String
The reason is that when the model method is called in the context of the unit test, open(self.public_filename) is returning a StringIO object that contains the image data. The path method on this object returns nil and MiniMagick::Image.from_file blows up.
When this very same model method is called from the PhotosController, open(self.public_filename) returns a FileIO instance tied to a file named, eg, /tmp/open-uri7378-0 and the file contains the image data.
Thinking the cause must be some environmental difference between test and development, I fired up the console under the development environment. But just as in the unit test, open('http://...') returned a StringIO, not a FileIO.
I've traced my way through open-uri and all the relevant application-specific code and can find no reason for the difference.