Mongoose populate vs aggregate

Question

I've noticed that .populate function in mongoose 4.7.3 runs separate queries on the database for each lookup:

  db.House
    .populate('ownerId')
    .exec((err, result) => {
    ..

With aggregation pipeline we can lookup multiple collections with a single query:

    db.House.aggregate([
    {
      $lookup:
      {
        from: 'owners',
        localField: 'ownerId',
        foreignField: '_id',
        as: 'owner',
      },

What is the reason for mongoose to do separate queries with .populate? Is the aggregation function more performant on lookups?

Answer 1

Here's a summary of the differences:

$lookup

• can only be used with aggregate
• can only be used to pull in referenced documents from unsharded collections. (Edit: as of MongoDB 5.1, $lookup can be used with sharded collections as well).
• can pull in referenced documents by any field
• generally more performant as it's a server-side operation
• requires MongoDB 3.2+

Mongoose populate()

• can be used with find and aggregate
• can be used to to pull in referenced documents from both sharded and unsharded collections
• can only pull in referenced documents by _id
• no MongoDB version requirement