AngularJS update problem

angular-app.js

var blogApp = angular.module('blogApp', [
  "ngRoute",
  "blogControllers",
  "blogFilters",
  "blogServices"]);

blogApp.config(['$routeProvider',
    function($routeProvider) {
      $routeProvider.
        when('/', {
          templateUrl: '/partials/posts',
          controller: 'PostController'
        }).
        when('/posts/:id', {
          templateUrl: '/partials/post',
          controller: 'ShowController'
        }).
        when('/admin', {
          templateUrl: '/partials/admin',
          controller: 'AdminController'
        }).
        when('/admin/update/:id', {
          templateUrl: '/partials/update',
          controller: 'AdminController'
        }).
        when('/admin/create', {
          templateUrl: '/partials/form',
          controller: 'AdminController'
        }).
        otherwise({
          redirectTo: '/'
        });
    }]);

api.js

var mongoose = require('mongoose');
var Post = mongoose.model('Post');

exports.update = function(req, res){
  var param = req.params.id;

  Post.findOne({_id: params}, function(err, post){
    post.title = req.body.title;
    post.content = req.body.content;
    post.save();
  });
};

controllers.js

(function() {
  'use strict';

  // ...
  blogControllers.controller('UpdateController', ["$scope", "PostsFactory", "$routeParams", "$location",
    function($scope, PostsFactory, $routeParams, $location){
      $scope.post = PostsFactory.get({'id': $routeParams.id}); 
      var post = $scope.post;
      var $id = post._id;

      $scope.update = function(){
        PostsFactory.update({_id: $id}, post);
        $location.path("/admin");
      };

    }]);

  // ...
})();

models-post.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var ObjectId = Schema.ObjectId;
var autoIncrement = require('mongoose-auto-increment');

var connection = mongoose.connect('mongodb://localhost/my-blog');

autoIncrement.initialize(connection);

var Post = new Schema({
  title: {type: String, required: true },
  content: {type: String, required: true},
  created_at: {type: Date, default: Date.now}
});

Post.plugin(autoIncrement.plugin, 'Book');
mongoose.model('Post', Post);

server-app.js

var model = require('./models/post.js');
var routes = require('./routes');
var api = require('./routes/api.js');
var express = require('express');
var http = require('http');
var path = require('path');
var app = express();

// all environments
app.set('port', process.env.PORT || 3000);
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');
app.use(express.favicon("public/images/favicon.ico"));
app.use(express.logger('dev'));
app.use(express.json());
app.use(express.urlencoded());
app.use(express.methodOverride());
app.use(express.cookieParser('your secret here'));
app.use(express.session());
app.use(app.router);
app.use(require('less-middleware')(path.join(__dirname, 'public')));
app.use(express.static(path.join(__dirname, 'public')));

// development only
if ('development' == app.get('env')) {
  app.use(express.errorHandler({ dumpExceptions: true, showStack: true}));
}

// Default routes
app.get('/', routes.index);
app.get('/partials/:filename', routes.partials);

// API
app.get('/api/posts', api.all);
app.post('/api/posts', api.create);
app.get('/api/posts/:id', api.show);
app.put('/api/posts/:id', api.update);
app.del('/api/posts/:id', api.remove);

http.createServer(app).listen(app.get('port'), function(){
  console.log('Magic on: ' + app.get('port'));
});

services.js

(function() {
  'use strict';

  var blogServices = angular.module('blogServices', ['ngResource']);

  blogServices.factory('PostsFactory', ["$resource",
    function($resource){
      return $resource('/api/posts/:id', {},
        {
          update: { method: 'PUT'},
          post: { method: 'POST'}
      });
    }]);
})();

update.jade

section(data-ng-controller="UpdateController")
  form(role="form", data-ng-submit="update()")
    fieldset
      .form-body
        .field
          label Title:
          input(type='text', data-ng-model='post.title')

        .field
          label Content:
          textarea(data-ng-model='post.content')

      .form-footer
        button(type="submit") Submit