MongoDB Lookup Aggregation Link to heading

The following will query for documents from a collection matching a id as a foreign id to a second collection.

db.mycollection1.aggregate([
  {
    $lookup: {
      from: "mycollection2",
      localField: "_id",
      foreignField: "_id",
      as: "matched_docs",
    }
  },
  {
    $match: {
      matched_docs: {$eq: []}
    }
  }
])