Model.aggregate()

Parameters

• [pipeline] «Array» aggregation pipeline as an array of objects

• [callback] «Function»

Returns:

• «Aggregate»

Performs aggregations on the models collection.

If a callback is passed, the aggregate is executed and a Promise is returned. If a callback is not passed, the aggregate itself is returned.

This function triggers the following middleware.

• aggregate()

Example:

// Find the max balance of all accounts
Users.aggregate([
  { $group: { _id: null, maxBalance: { $max: '$balance' }}},
  { $project: { _id: 0, maxBalance: 1 }}
]).
then(function (res) {
  console.log(res); // [ { maxBalance: 98000 } ]
});
// Or use the aggregation pipeline builder.
Users.aggregate().
  group({ _id: null, maxBalance: { $max: '$balance' } }).
  project('-id maxBalance').
  exec(function (err, res) {
    if (err) return handleError(err);
    console.log(res); // [ { maxBalance: 98 } ]
  });

NOTE:

• Mongoose does not cast aggregation pipelines to the model’s schema because $project and $group operators allow redefining the “shape” of the documents at any stage of the pipeline, which may leave documents in an incompatible format. You can use the mongoose-cast-aggregation plugin to enable minimal casting for aggregation pipelines.
• The documents returned are plain javascript objects, not mongoose documents (since any shape of document can be returned).

More About Aggregations:

• Mongoose Aggregate
• An Introduction to Mongoose Aggregate
• MongoDB Aggregation docs