AngularJS: Version with global variables.

controllers.js

app.controller('mainCtrl', function($rootScope, UserService) {
  $rootScope.users = [];
  $rootScope.todos = [];

  // Get all users.
  UserService.all().then(function(users) {
    angular.forEach(users, function(user) {
      $scope.users.push(user);

      // Get all todos.
      TodoService.all(team.id, user.id).then(function(todos) {
        angular.forEach(todos, function(todo) {
          $rootScope.todos.push(todo);
        });
      });
    });
  });
});

app.controller('UserCtrl', function($rootScope, $scope) {
  // Easy to change $rootScope.todos, and it will affect everywhere automatically.
  $scope.changeOwner = function(todoId, userId) {
    angular.forEach($rootScope.todos, function(todo) {
      if (todo.id == todoId) {
        todo.userId = userId;
      }
    });
  };
});

index.html

<div ng-controller="mainCtrl">
  <div ng-repeat="user in users">
    <ul ng-controller="UserCtrl">
      <li ng-repeat="todo in todos | filter:{userId: user.id}">
        {{todo.title}}
      </li>
    </ul>
  </div>
</div>

services.js

app.service('UserService', function($http, $q) {
  this.all = function() {
    var deferred = $q.defer();

    $http.get('/api/users/').success(function(users) {
      return deferred.resolve(users);
    });

    return deferred.promise;
  };
});

app.service('TodoService', function($http, $q) {
  this.all = function(userId) {
    var deferred = $q.defer();

    $http.get('/api/users/' + userId + '/todos').success(function(todos) {
      return deferred.resolve(todos);
    });

    return deferred.promise;
  };
});