java mongo driver 3.0+

问题:

Using the new 3.0+ java driver from mongo what is the best way to check if a document exists in a collection.

I have looked at here and tried to do something similar. I have only gotten as far as this:

FindIterable<Document> iterable = collection.find(eq("code", "abcdefg")).projection(Projections.include("_id")).limit(1);

This returns a FindIterable but how do you check it has found anything at all ? If you can please provide a code example.

I did try:

if (!iterable.first().isEmpty()){System.out.println(" RESILT IS FOUND ");}else{System.out.println(" RESULT IS NOT FOUND ");}

but when the query returns nothing it dies with the following error:

Exception in thread "main" java.lang.NullPointerException
    at com.oss.niagaramqtt.MongoLib.exists(MongoLib.java:58)
    at com.oss.niagaramqtt.MongoLib.<init>(MongoLib.java:47)
    at com.oss.niagaramqtt.startup.main(startup.java:24)

Indeed is this the correct approach overall for checking the existence of a document?

EDIT: This could be the answer please confirm:

MongoCursor<Document> iterable = collection.find(eq("code", "abcdefg")).projection(Projections.include("_id")).limit(1).iterator();                
if (iterable.hasNext()){System.out.println(" RESILT IS FOUND ");}else{System.out.println(" RESULT IS NOT FOUND ");}

回答1:

Your way is good if you need to load this document in case it exists. If you don't need to load it then you can use MongoCollection.count method like:

    long count = collection.count(new BsonDocument("code", new BsonString("abcdefg")));
    if (count > 0){System.out.println(" RESILT IS FOUND ");}else{System.out.println(" RESULT IS NOT FOUND ");}

[Update] In case data is stored on a sharded cluster, db.collection.count() can result in an inaccurate count if orphaned documents exist or if a chunk migration is in progress. So it's safer to use aggregate function instead:

    Iterator<Document> it = collection.aggregate(Arrays.asList(
            new Document("$match", new Document("code", "abcdefg")),
            new Document("$group", new Document("_id", null).append("count", 
                    new Document("$sum", 1))))).iterator();
    int count = it.hasNext() ? (Integer)it.next().get("count") : 0;

See http://docs.mongodb.org/manual/reference/sql-aggregation-comparison/ for more details.

回答2:

Please check this count operation at java API 3.0 in a collection.

You can basically do:

   MongoClient mongoClient2 = new MongoClient("localhost", 27017);
   long count =  mongoClient2.getDatabase("mongoBase")               
                             .getCollection("collection")
                             .count(new Document("code", "abcdefg"));
   if(count>0){
   //exists 
   }

you will find great examples here:

https://github.com/mongodb/mongo-java-driver/tree/8866de115d45ea73a39da9360159d3fbb6390dd9/driver/src/examples/tour