Testing Middleware with Rspec

Question

I've written some Rack-Middleware and now I'm trying to test it with Rspec. But all Rack-Middleware is instantiated with an 'app' argument, that represents the Rails app itself. How do you guys mock this up in Rspec?

For example,

 describe MyMiddleWare do     let(:app) { # How do I mock a Rails app object here? }     subject { MyMiddleWare.new(app: app) }      it 'should blah blah blah' do        # a bunch of tests go here     end  end 

Answer 1

You just need the world's simplest Rack app:

let(:app) { lambda {|env| [200, {'Content-Type' => 'text/plain'}, ['OK']]} } 

Also, your middleware's constructor should receive an app as its first parameter not a hash so it should read:

subject { MyMiddleWare.new(app) } 

In all likelihood, though, your test is going to need to determine what effect the middleware has had on the request. So you might write a slightly more sophisticated rack app to spy on your middleware.

class MockRackApp    attr_reader :request_body    def initialize     @request_headers = {}   end    def call(env)     @env = env     @request_body = env['rack.input'].read     [200, {'Content-Type' => 'text/plain'}, ['OK']]   end    def [](key)     @env[key]   end  end 

and then you'll probably want to use Rack::MockRequest to actually send the request. Something like:

describe MyMiddleWare do    let(:app) { MockRackApp.new }   subject { described_class.new(app) }    context "when called with a POST request" do     let(:request) { Rack::MockRequest.new(subject) }     before(:each) do       request.post("/some/path", input: post_data, 'CONTENT_TYPE' => 'text/plain')     end      context "with some particular data" do       let(:post_data) { "String or IO post data" }        it "passes the request through unchanged" do         expect(app['CONTENT_TYPE']).to eq('text/plain')         expect(app['CONTENT_LENGTH'].to_i).to eq(post_data.length)         expect(app.request_body).to eq(post_data)       end     end   end end