Mongo: find items that don't have a certain field

Question

How to search for documents in a collection that are missing a certain field in MongoDB?

Answer 1

If you don't care if the field is missing or null (or if it's never null) then you can use the slightly shorter and safer:

db.things.find( { a : null } ); // return if a is missing or null

It's safer because $exists will return true even if the field is null, which often is not the desired result and can lead to an NPE.