app.factory('Tarefa',
  function($resource){
    return $resource('https://todowebservice.herokuapp.com/api/todos/:id');
  });

app.controller('tarefaController',
  function($scope, $routeParams, Tarefa){

    if($routeParams.id){
      Tarefa.get(
        {id: $routeParams.id},
        function(tarefa){
          $scope.tarefa = tarefa;
        },
        function(erro){
          console.log(erro);
        });
    }else{
      $scope.tarefa = new Tarefa();
    }

    $scope.salva = function(){
      $scope.tarefa.$save()
      .then(function(){
        $scope.tarefa = new Tarefa();
      })
      .catch(function(erro){
        console.log(erro);
      });
    };
  });

<form ng-submit="salva()">
    <div class="form-group">
        <label for="description">Descrição</label>
        <input type="text" name="description" id="description" class="form-control" required ng-model="tarefa.description" />
    </div>
    <div class="form-group">
        <label for="done">Realizado</label>
        <div class="input-group">
            <input type="checkbox" class="form-control" id="done" name="done"
                   ng-model="tarefa.done" />
        </div>
    </div>
    <div class="form-group">
        <label for="createdAt">Criado em</label>
        <input type="text" name="createdAt" id="createdAt" class="form-control" ng-model="tarefa.createdAt" />
    </div>
    <div>
        <button type="submit" class="btn btn-primary">
            Salvar
        </button>
    </div>
</form>

{…}​config: Object { method: "POST", transformRequest: (1) […], url: "https://todowebservice.herokuapp.com/api/todos", … }​data: Object { code: 11000, index: 0, errmsg: "E11000 duplicate key error index: todoweb.todos.$_id_ dup key: { : ObjectId('5ab30cbded34a40011eb7be6') }", … }​headers: function headersGetter()​status: 400​statusText: "Bad Request"​__proto__: Object { … } tarefaController.js:23:9