Get “data from collection b not in collection a” in a MongoDB shell query

Question

I have two MongoDB collections that share a common _id. Using the mongo shell, I want to find all documents in one collection that do not have a matching _id in the other co

Answer 1

In mongo 3.2 the following code seems to work

db.collectionb.aggregate([
    {
      $lookup:
        {
          from: "collectiona",
          localField: "collectionb_fk",
          foreignField: "collectiona_fk",
          as: "matched_docs"
        }
   },
   {
      $match: { "matched_docs": { $eq: [] } }
   }
]);

based on this https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#use-lookup-with-an-array example