angularJs TypeError: Converting circular structure

I have a small AngularJs app like so:

 // html

 <div ng-repeat="project in projects">
     <h3>{{ project.id }}</h3>
     <h3>{{ project.title }}</h3>
      <div ng-repeat="task in project.tasks">
          <h4>{{ task.id }}. {{ task.title }}</h4>
          <button class="btn btn-default" ng-click="showEditTask=true">Edit Task</button>
          <div class="box row animate-show-hide" ng-show="showEditTask">

              <h2>Create a New Task</h2>
              <form name="newTaskForm" class="form-horizontal">
                  Title: <br />
                  <input ng-model="new_task.title" type="text" id="title" name="title" class="form-control" placeholder="Title" required /><br />

                  Project: <br />
                  <select ng-model="new_task.project" ng-options="project.title for project in projects" class="form-control"></select><br>
              </form>

                  <button class="btn btn-success" ng-click="createTask(new_task)" ng-disabled="!newTaskForm.title.$valid">create</button>
          </div>
      </div>
 </div>


  // app.js

  concernsApp.factory('ConcernService', function ($http, $q) {

    ...

    update: function (obj_url, obj) {
        var defer = $q.defer();
        console.log(obj)
        $http({method: 'POST',
            url: api_url + obj_url + obj.id + '/',
            data: obj}).
            success(function (data, status, headers, config) {
                defer.resolve(data);
            }).error(function (data, status, headers, config) {
                defer.reject(status);
            });
        return defer.promise;
     },
   });


 concernsApp.controller('ProjectsCtrl', function ($scope, $http, ConcernService) {

    $scope.updateTask = function(obj) {
        ConcernService.update('tasks/', obj).then(function(){
        ...
    }
 });

The problem is with updating a task and leaving the parent project as is. If I change the parent project, it all works fine. If I use the same parent project then I get:

TypeError: Converting circular structure to JSON

I'm not entirely sure what is happening here.

Edit

So, I can solve the problem like this:

$scope.updateTask = function(obj) {
    parentProject = {'id': obj.project.id};
    obj.project = parentProject;
    ConcernService.update('tasks/', obj).then(function(){
        ...
    });
};

This works as I only actually need the task.project.id to update the object. I think the problem is due to that fact that the task references a parent project, which in turn references the child tasks etc. I'm not entirely sure about this tough.

However, this solution seems a little hacky to me and I would love to see a better solution.

Somewhere you are trying to convert an object to JSON, and that has a circular reference in it, meaning something like this:

var myObj = new Object();
myObject.reference = myObj;

Now trying to convert this to JSON will fail, and produce your error.

Looking at the your edited post, your data structure is something like this:

task.parent -> {project}

project.tasks -> [{task1},{task2},...]

This is supposed to model the relation "project has multiple tasks" and "task belongs to a project".

Now this can obviously not be converted to json format, due to circular referencing.

For the data structure, why not go with:

project = { "id": 42,
            "tasks": [123, 124, 127, ... ],
          }

task = { "id": 123,
         "idproject": 42,
            ...
       }

Whenever you have to show the relationships in your angularjs app, you could go with filters. If you want to show what belongs to a project, filter all tasks by projectid.

Or retrieve only the required data from your backend on the fly, by requesting what tasks the backend has for a specific project id.

That is one way to do it. Of course, having the backend accept appropriate update requests would be working just as well, e.g. updating a project would require the project id, and an array of task ids.