See also https://docs.mongodb.com/manual/reference/operator/query-element/ .

We want to test the shape of the documents.

$exists operator
----------------

Check if a field exists or not.

In our database there are some old movies too, and thus they have
no Tomato score.

First, find movies that have a Tomato meter:

    db.movieDetails.find({"tomato.meter": {$exists: true}}).pretty()
    db.movieDetails.find({"tomato.meter": {$exists: 1}}).pretty()       // same


Now find movies that have no Tomato meter score:

    TODO...