Where is fixture_file_upload defined? Can you try Kernel.fixture_file_upload?

Okay so it looks like my project isn't loading the test_helper.rb because when I change the code to the following it will work

include ActionDispatch::TestProcess
FactoryGirl.define do
    factory :mongo_document do 
      original_filename "test.jpg"
      file { fixture_file_upload(Rails.root.to_s + '/spec/fixtures/files/test.jpg', 'img/jpeg') }
  end
end

Thank you @batreyud, it works for me adding "include ActionDispatch::TestProcess".

Thanks for solution.It helps me also for "fixture_file_upload" in Rspec model.

Be careful, I've had some rather obscure, session related problems when I included the module. See: http://stackoverflow.com/questions/18202261/include-actiondispatchtestprocess-prevents-guard-from-reloading-properly.

Don't include ActionDispatch::TestProcess. It has some nasty consequences.

Proper solution is to use Rack::Test::UploadedFile directly in the factory (what fixture_file_upload essentially does anyway):

file { Rack::Test::UploadedFile.new('spec/factories/test.png', 'image/png') }

@pjg I've been trying to use Rack::Test::UploadedFile, but I get an error REXML::UndefinedNamespaceException: Undefined prefix Rack::Test: found. Do I need to require a file or include a module to make this work?

@josephbhunt Judging by the REXML::UndefinedNamespaceException exception it looks like you're in the wrong context when you execute this file helper (do you have it in the factory definition?). As a workaround you might try to prefix Rack with :: so that it references top-level namespace and becomes ::Rack::Test::UploadedFile.