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

Yeah, it's possible using $exists:

db.things.find( { a : { $exists : false } } ); // return if a is missing

When is true, $exists matches the documents that contain the field, including documents where the field value is null. If is false, the query returns only the documents that do not contain the field.