how to call count operation after find with mongodb java driver

Question

I am using MongoDB 3.0. suppose there is a set of documents named photos, its structure is

{"_id" : 1, photographer: "jack"}

with database.getCollection("photos"), Mongodb will return a MongoCollection object, on which I have the method count() to get the number documents returned.

However, when I make queries with specific conditions. For example find documents with id smaller than 100 :

photosCollections.find(Document.parse("{_id : {$lt : 100}}"))

Above find method will always return a cursor which doesn't provide a count() function. So how can I know how many documents returned ? I know on command line, I can use

db.photos.find({_id : {$lt : 100}}).count()

Of course, I can go through the iterator and count the number of documents myself. However I find it really clumsy. I am wondering does MongoDB java driver provides such functionality to count the number of documents returned by the find() method ? If not, what is the reason behind the decision ?

Answer 1

As you said the MongoCollection has the count() method that will return the number of documents in the collection, but it has also a count(Bson filter) that will return the number of documents in the collection according to the given options.

So you can just use:

long count = photosCollections.count(Document.parse("{_id : {$lt : 100}}"))

or maybe clearer:

Document query = new Document("_id", new Document("$lt", 100));
long count = photosCollections.count(query);

ref: http://api.mongodb.com/java/3.3/com/mongodb/client/MongoCollection.html#count-org.bson.conversions.Bson-