MongoDB Java Driver creating Database and Collection

Question

i was testing how to create database and collection mongo java driver.

  MongoClient client = new MongoClient("localhost",27017);
        DB db = client.getDB("ow");
        DBCollection collection = db.getCollection("documents");
        collection.save(new BasicDBObject("_id",1));
        collection.remove(new BasicDBObject("_id",1));
        boolean result = db.collectionExists("documents");
        assertTrue(result);
        assertNotNull(collection);
        client.close();

I would prefer to use createCollection method on the DB object, but found that it does not create database / collection unless the first document is inserted.

My question is is this understanding correct ? Is above code correct was of creating collection or database.

BatScream · Accepted Answer · 2014-11-16 02:14:48Z

5

prefer to use createCollection method on the DB object, but found that it does not create database / collection unless the first document is inserted.

MongoDB creates a collection implicitly when the first document is saved into a collection. The createCollection() method explicitly creates a collection only and only if an options object is passed to it as an argument.

Now this makes sense. The options parameter can take in one or more arguments to decide the characteristics of the collection we want to create such as capped,autoIndexId,size,usePowerOf2Sizes,max no. of documents.

If we do not specify any of these options, the default behavior would take precedence, i.e create a collection lazily whenever the first insert is made, with default settings.

So if we want a collection whose characteristics we are going to define, then we can pass these characteristics as a DBObject to the createCollections() method and our collection would be created. Below is an example of how to pass the options.

BasicDBObject options =  new BasicDBObject();
options.put("size", 12121212);
db.createCollection("hello", options);

Is above code correct was of creating collection or database.

Yes. It allows mongodb to apply the default configuration for your collection. Unless you want to set the max,size,autoIndexId,capped,usePowerOf2Sizes properties for your new collection, this is fine.

Refer: http://docs.mongodb.org/manual/reference/method/db.createCollection/

edited Nov 16, 2014 at 2:14

answered Nov 16, 2014 at 2:08

BatScream

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2 Comments

user2775185 Over a year ago

Thanks. Concern is after createCollection method before we insert any document in mongo, show dbs does not display the db / collection, which means that creation collection which does not have any document before hand does not make any sense. right ?

BatScream Over a year ago

Yes. MongoDB need not create an empty collection without knowing if the client is going to use it at all. If it did it would have been a overhead. But when the client specifically requests for a collection with options, then it does create the collection.

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