mongodb query promise sample

http://mongoosejs.com/docs/api.html#promise_Promise
Promise#then(onFulFill, onReject)

Creates a new promise and returns it. If onFulfill or onReject are passed, they are added as SUCCESS/ERROR callbacks to this promise after the nextTick. Parameters: onFulFill <Function> onReject <Function> Returns: <Promise> newPromise See: promises-A+ mpromise#then

Conforms to promises/A+ specification. Example:

var promise = Meetups.find({ tags: 'javascript' }).select('_id').exec();promise.then(function (meetups) {  var ids = meetups.map(function (m) {    return m._id;  });  return People.find({ meetups: { $in: ids }).exec();}).then(function (people) {  if (people.length < 10000) {    throw new Error('Too few people!!!');  } else {    throw new Error('Still need more people!!!');  }}).then(null, function (err) {  assert.ok(err instanceof Error);});