How to check if a field is not null in mongodb?

Use following to get all documents with both a key called "myfield" and a non-null value:

db.mycollection.find({"myfield":{$ne:null}});

Use following to get all documents with a key called "myfield", but they may still have a null value:

db.mycollection.find({"myfield":{$exists:true}});

Reference: https://docs.mongodb.com/manual/tutorial/query-for-null-fields/