$lookup on ObjectId's in an array

Question

What\'s the syntax for doing a $lookup on a field that is an array of ObjectIds rather than just a single ObjectId?

Example Order Document:

{
  _id:          


        

Answer 1

Starting with MongoDB v3.4 (released in 2016), the $lookup aggregation pipeline stage can also work directly with an array. There is no need for $unwind any more.

This was tracked in SERVER-22881.