Today I Learned

This aggregation stage performs a left outer join to a collection in the same database.

There are four required fields:

from: The collection to use for lookup in the same database
localField: The field in the primary collection that can be used as a unique identifier in the from collection.
foreignField: The field in the from collection that can be used as a unique identifier in the primary collection.
as: The name of the new field that will contain the matching documents from the from collection.

Example:

db.comments.aggregate([
  {
    $lookup: {
      from: "movies",
      localField: "movie_id",
      foreignField: "_id",
      as: "movie_details",
    },
  },
  {
    $limit: 1
  }
])