AngularJS - wait for multiple resource queries to complete

I have a single factory defined with ngResource:

App.factory('Account', function($resource) {
    return $resource('url', {}, {
        query: { method: 'GET' }
    });
});

I am making multiple calls to the query method defined on this factory. The calls can happen asynchronously, but I need to wait for both calls to complete before continuing:

App.controller('AccountsCtrl', function ($scope, Account) {
    $scope.loadAccounts = function () {
        var billingAccounts = Account.query({ type: 'billing' });
        var shippingAccounts = Account.query({ type: 'shipping' });

        // wait for both calls to complete before returning
    };
});

Is there a way to do this with AngularJS factories defined with ngResource, similar to jQuery's $.when().then() functionality? I would prefer not to add jQuery to my current project.

You'll want to use promises and $q.all().

Basically, you can use it to wrap all of your $resource or $http calls because they return promises.

function doQuery(type) {
   var d = $q.defer();
   var result = Account.query({ type: type }, function() {
        d.resolve(result);
   });
   return d.promise;
}

$q.all([
   doQuery('billing'),
   doQuery('shipping')
]).then(function(data) {
   var billingAccounts = data[0];
   var shippingAccounts = data[1];

   //TODO: something...
});

I think a better solution is:

$q.all([
   Account.query({ type: 'billing' }).$promise,
   Account.query({ type: 'shipping' }).$promise
]).then(function(data) {
   var billingAccounts = data[0];
   var shippingAccounts = data[1];

   //TODO: something...
});

The solution from Ben Lesh is the best but it's not complete. If you need to handle error conditions--and, yes, you do--then you must use the catch method on the promise API like this:

$q.all([
   doQuery('billing'),
   doQuery('shipping')
]).then(function(data) {
   var billingAccounts = data[0];
   var shippingAccounts = data[1];

   //TODO: something...

}).catch(function(data) {

   //TODO: handle the error conditions...

}).finally(function () {
  
  //TODO: do final clean up work, etc...

});

If you don't define catch and all of your promises fail, then the then method won't ever execute and thus will probably leave your interface in a bad state.