rack_test_middleware.rb

require 'rack/test'

describe SampleMiddleware do
    include Rack::Test::Methods

    let(:inner_app) do
        lambda { |env| [200, {'Content-Type' => 'text/plain'}, ['All good!'] }
    end

    let(:app) { SampleMiddleware.new(inner_app) }

    it "adds hello:world to session" do
        get "/"
        last_request.session['hello'].should == 'world'
    end

    it "makes no change to response status" do
        get "/"
        last_response.should be_ok
    end
end

class SampleMiddleware
    def initialize(app)
        @app = app
    end
    def call(env)
        request = Rack::Request(env)
        request.session['hello'] = 'world'
        @app.call(env)
    end
end

some_django_middleware.py

class SomeMiddleware(object):
    def process_request(self, request):
        # alter the request in some way
        return None   # or return a response
    def process_response(self, request, response):
        # alter the response in some way
        return response

some_rack_middleware.rb

class SomeMiddleware
    def initialize(app)
        @app = app
    end
    def call(env)
        request = Rack::Request(env)
        # alter the request in some way
        response = @app.call(env)
        # alter the response in some way
        return response
    end
end