unable to send post request on the server using backbone.js

Question

I am making post request on the server with an id the request looks like this:

 app.post('campaign/update/:_id', campaign.update);    

the save method is this which calls post request:

 this.model.save(null, {success: function(data){
            });
        }});

and the model is:

var CampaignEditModel = Backbone.Model.extend({
    urlRoot:"http://localhost:3033/campaign/update/",
     url : function(){
     var url = this.urlRoot + this.id;
      console.log("in model:"+ this.id ) ;
      return url;
  },
    idAttribute: "_id",
    defaults:{
        "id":"",
        "Name" :""
    }
});

the request goes like this:

 http://localhost:3033/campaign/update/5c72e198-c8fd-11e2-9cfa-74867a028324

the problem is the console shows the error:

 Failed to load resource: the server responded with a status of 404 (Not Found)