How to remove a field completely from a MongoDB document ?

just try to do this below code:

db.example.update({}, {$unset: {words:1}}, false, true);

further information http://www.mongodb.org/display/DOCS/Updating#Updating-%24unset

UPDATING:

For the link https://docs.mongodb.com/manual/reference/operator/update/unset/no longer covers ‘$unset’ing. Be sure to add {multi: true} if you want to delete this field from entire of the documents in the collection; or else, it will only delete it from the first document it search that matches. See this for updated documentation:

Specimen:

db.example.update({}, {$unset: {words:1}} , {multi: true});

To delete field in MongoDB:

The lonely record:

db.getCollection('userData').update({}, {$unset: {pi: 1}})

The many record:

db.getCollection('userData').update({}, {$unset: {pi: 1}}, {multi: true})

you can also just try to do this one:

db.example.updateMany({},{"$unset":{"tags.words":1}})