mongoose test post middleware

model.js

var Page = new Schema({
    title: { type: String, default: '', trim: true },
    url:   { type: String, default: '', trim: true },
    thumbnails: {type: Schema.ObjectId,  ref: 'Thumbnails'},
  });

  Page.post('save', function(doc) {
   if(doc.thumbnails) return;
   Thumbnail.generate(doc.url, function(err, thumb){
     doc.thumbnails = thumb;
     doc.save()
   });
  });

 var Thumbnail = new Schema({
    domain: {type: String, trim: true},
    url: {
      small: { type: String, trim: true },
      original: {type: String, trim: true }
    }
 });

 Thumbnail.statics.generate = function(doc, cb) {
   // generate site preview , save the thumbnail and call cb
 }

model_test.js

var Page = mongoose.model('Page');
var Thumbnail = mongoose.model('Thumbnail');

// How to verify post middleware ?

it("generates thumbnail if page does not have one", function(done){
   var pageAttr = { title: 'Gist web page', url: 'http://www.gist.github.com' }
   var gist = new Page(pageAttr), function(err, _page){
     if(err) throw err;
     done();
   })