How to know about the Keys

"Chapter 2: The MongoDB Query Language + Atlas: Updating Documents: updateOne() ",

How can we find an entry with lesser number of keys than others?
(How to know if there is an entry with title : The Martian having keys lesser than the usual?)

I believe we’ve looked into this before and there is no quick and easy way.

If you want a specific field to apply to each and every document, then you can first search for all documents where that field has value “null”. Then you can use the results to update the docs to include the field with your desired value.