Improve PlaylistsController POST create specs

spec/controllers/playlists_controller_spec.rb

@@ -29,36 +29,25 @@ describe PlaylistsController do
   end
 
   describe 'POST create' do
-    let(:playlist) { mock_model(Playlist).as_null_object }
-    before { Playlist.stub(:new).and_return(playlist) }
-
+    context 'whith valid params' do
       it 'creates a new playlist' do
-      Playlist.should_receive(:new).
-        with(Factory.attributes_for(:playlist)).
-        and_return(playlist)
+        expect {
           post :create, :playlist => Factory.attributes_for(:playlist)
+        }.to change(Playlist, :count).by(1)
       end
 
-    it 'saves the playlist' do
-      playlist.should_receive(:save)
-      post :create, :playlist => {}
-    end
-
-    context 'when the playlist saves successfully' do
       it 'redirects to the playlists index' do
-        post :create, :playlist => {}
+        post :create, :playlist => Factory.attributes_for(:playlist)
         response.should redirect_to(:action => 'index')
       end
     end
 
-    context 'when the playlist fails to save' do
-      before do
-        playlist.stub(:save).and_return(false)
-      end
+    context 'whith invalid params' do
+      before { Playlist.any_instance.stub(:save).and_return(false) }
 
       it 'assigns the playlist as @playlist' do
         post :create, :playlist => {}
-        assigns[:playlist].should eq(playlist)
+        assigns[:playlist].should be_a_new(Playlist)
       end
 
       it 'renders the new template' do