collection 中如果包含两种doc，find 无法完全匹配key

想查找colletion中所以UDID的值？但是collection 混入了其他的doc导致查询到其他不包含UDID key的值，请问应该如何来写这个查询？如何避免collection混如其他的doc

 MongoClient.connect("mongodb://localhost:27017/mydb", function(err, db) {
        if(!err) {
            console.log("We are connected");
            var collection = db.collection('test');


            collection.find({"UDID":{"$ne":"a"}},{"_id":0}).toArray(function(err, items) {

                console.log(items);

            });

        }

    });

输出

We are connected
[ { str: 'foobar' },
  { str: 'foobar' },
  { str: 'foobar' },
  { hello: 'doc1' },
  { hello: 'doc1' },
  { UDID: 'asdajsdajkshdkjasdasdasdas' },
  { UDID: 'asdajsdajkshdkjasdasdasdas' },
  { UDID: 'asdajsdajkshdkjasdasdasdas' } ]

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$exist 查询符