express aggregate in mongoose code example

Example 1: mongoose aggregate

const aggregate = Model.aggregate([
  { $project: { a: 1, b: 1 } },
  { $skip: 5 }
]);

Model.
  aggregate([{ $match: { age: { $gte: 21 }}}]).
  unwind('tags').
  exec(callback);

Example 2: how to use mongoose aggregate

new Aggregate([{ $match: { _id: '00000000000000000000000a' } }]);
  // Do this instead to cast to an ObjectId
  new Aggregate([{ $match: { _id: mongoose.Types.ObjectId('00000000000000000000000a') } }]);