最后找到这个，它表明尽管由 fixture_file_upload 返回的东西有一个 @tempfile 成员，缺少读者的方法。解决如下：

  FileUtils.touch（'file.zip'）＃fixture_file_upload需要文件存在
 @file = fixture_file_upload（'file.zip'）
 class<< @file 
＃reader方法存在于实际调用中，
＃但是由于某种原因从fixture对象中缺少（Rails 3.1.1）
 attr_reader：tempfile 
 end 
 

My controller accesses the tempfile attribute of an uploaded file and passes it to another mocked component. My test code has

  @file = mock(Object)
  @file.stub_chain(:tempfile, :path).and_return('thefile.zip')
  # ...
  post :create, :file => @file

and the controller code calls params[:file].tempfile.path.

After upgrading from Rails 3.0 to 3.1, the above line started failing with

undefined method `tempfile' for "#[RSpec::Mocks::Mock:0x2b0d9a0 @name=Object]":String

That is, Rails 3.1 converted params[:file] to a string automatically.

The code works properly when tested manually through a browser. I tried to use fixture_file_upload and the parameter became a File object but it had no tempfile method.

So how do I pass an arbitrary mock object as a parameter to an action in Rails 3.1?

Finally found this, which tells that although the thing returned by fixture_file_upload has a @tempfile member, it lacks the reader method. Solved as follows

  FileUtils.touch('file.zip') # fixture_file_upload needs the file to exist
  @file = fixture_file_upload('file.zip')
  class << @file
    # The reader method is present in a real invocation,
    # but missing from the fixture object for some reason (Rails 3.1.1)
    attr_reader :tempfile
  end