Update data in mongoDB with Moongoose and AngularJs

I'm using NodeJS, AngularJS, and MongoDB with mongoose to make a website. I'm having some trouble for modify an object in mongoDB. I follow a tutorial and everything work with the exemple. But I try with another table, the name of the new table is post and I have a problem when I want to change a value in the table.

Here is my Model :

var express = require('express');
var router = express.Router();
var mongoose = require('mongoose');
var Todo = require('../models/Post.js');

    var PostSchema = new mongoose.Schema({
        nomReseau : String,
        corps : String,
        etat : String,
        section : String
    });
var Post = mongoose.model('PostReseaux', PostSchema);
module.exports = Post;

Here you can see the html :

 <ul>
    <li ng-repeat="post in posts ">
        <div class="card blue-grey darken-1">
            <div class="card-content white-text">
                <span class="card-title">Ticket</span>
                <p>
                    <a ng-show="!editing[$index]" href="#/{{post._id}}">{{post.corps}}</a>
                </p>
                <input type="checkbox" ng-model="post.etat" ng-change="update($index)">
                <button ng-show="!editing[$index]" ng-click="edit($index)">Editer</button>
                <button ng-show="!editing[$index]" ng-click="remove($index)">Supprimer</button>
                <button ng-show="!editing[$index]" ng-click="passer($index)">Passer</button>

                <input ng-show="editing[$index]" type="text" ng-model="post.corps">
                <button ng-show="editing[$index]" ng-click="update($index)">Confirmer</button>
                <button ng-show="editing[$index]" ng-click="cancel($index)">Annuler</button>
            </div>
        </div>
    </li>
</ul>
<a class="waves-effect waves-light btn" ng-click="save()">Créer ticket</a>

It's a simple ng-repeat who show every post in my database, if I use the remove or the save functions everything works I can add or remove a post in the db.

Here is the controller :

angular.module('app').controller('Feedback', ['$scope','$location','Posts', function($scope, $location, Posts) {
    $scope.title = 'Feedbacks';
    $(".button-collapse").sideNav();
    $scope.editing = [];
    $scope.posts= Posts.query();

    $scope.save = function(){
        if($scope.CorpsTicket.length < 1) return;
        var post = new Posts({ nomReseau: "Google+", corps : "test", etat : "aTraiter", section :"feedback" });

        post.$save(function(){
            $scope.posts.push(post);
        });
    }

    $scope.update = function(index){
        var post = $scope.posts[index].etat;
        Posts.update({id: post._id}, post);
        $scope.editing[index] = false;
    }

    $scope.passer = function(index){
        var post = $scope.posts[index];
        post.etat = "enCours";
        Posts.update({id: post._id}, post);
        $scope.editing[index] = false;
    }

    $scope.edit = function(index){
        $scope.editing[index] = angular.copy($scope.posts[index]);
    }

    $scope.cancel = function(index){
        $scope.posts[index] = angular.copy($scope.editing[index]);
        $scope.editing[index] = false;
    }

    $scope.remove = function(index){
        var post = $scope.posts[index];
        Posts.remove({id: post._id}, function(){
            $scope.posts.splice(index, 1);
        });
    }


}])

And here is the js for the backend :

var express = require('express');
var router = express.Router();

var mongoose = require('mongoose');
var Post = require('../models/Post.js');

/* GET /post listing. */
router.get('/', function(req, res, next) {
    Post.find(function (err, posts) {
        if (err) return next(err);
        res.json(posts);
    });
});


/* POST /post */
router.post('/', function(req, res, next) {
    Post.create(req.body, function (err, post) {
        if (err) return next(err);
        res.json(post);
});
});

/* GET /todos/id */
router.get('/:id', function(req, res, next) {
    Post.findById(req.params.id, function (err, post) {
        if (err) return next(err);
        res.json(post);
    });
});


/* PUT /todos/:id */
router.put('/:id', function(req, res, next) {
    Post.findByIdAndUpdate(req.params.id, req.body, function (err, post) {
        if (err) return next(err);
        res.json(post);
    });
});

/* DELETE /todos/:id */
router.delete('/:id', function(req, res, next) {
    Post.findByIdAndRemove(req.params.id, req.body, function (err, post) {
        if (err) return next(err);
        res.json(post);
    });
});

module.exports = router;

But if I click on edit($index) and I change the value of post.corps and I click on Confirmer the value change in the screen but not change in the database.

I think I have a problem with the update function, but I don't know what it is. Like is say, I have done a tutorial and, with the table create in the tutorial, everything works I can modify a value, but I got this bug with this new Table post.

Thanks