mongodb怎么删除所有为空的字段

//run in mongo shell

var coll = db.getCollection("collectionName");

var cursor = coll.find();

while (cursor.hasNext()) {

 var doc = cursor.next();

 var keys = {};

 var hasNull = false;

 for ( var x in doc) {

 if (x!="_id" && doc[x] == null) {

 keys[x] = 1;

 hasNull = true;

 }

 }

 if (hasNull) {

 coll.update({_id: doc._id}, {$unset:keys});

 }

}