Query a document on array elements in MongoDB using Java

Question

I am new to MongoDB. My sample document is

{
    "Notification" : [
        {
            "date_from" : ISODate("2013-07-08T18:30:00Z"),
            "date_too" : ISODate("2013-07-30T18:30:00Z"),
            "description" : "fdfd",
            "url" : "www.adf.com"
        },
        {
            "date_from" : ISODate("2013-07-01T18:30:00Z"),
            "date_too" : ISODate("2013-07-30T18:30:00Z"),
            "description" : "ddddddddddd",
            "url" : "www.pqr.com"
        }
    ],

I am trying to update the Notification whose "url" : "www.adf.com". My Java code to do this is:

BasicDBObject query=new BasicDBObject("url","www.adf.com");

DBCursor f = con.coll.find(query);

It does not search for the document whose "url" is "www.adf.com".

You forgot the Notification in your query. You have to query for Notification.url. — obecker
– obecker, Commented Jul 26, 2013 at 6:46
@obecker Notification is array not nested doc. I also tried it BasicDBObject query=new BasicDBObject("Notification.url","www.adf.com"); — Prashant Thorat
– Prashant Thorat, Commented Jul 26, 2013 at 9:02
see docs.mongodb.org/manual/core/read-operations -> Arrays The nested object syntax should work also for arrays. — obecker
– obecker, Commented Jul 26, 2013 at 9:08

Philipp · Accepted Answer · 2013-07-26 12:46:06Z

14

You have a nested document in this case. Your document has a field Notification which is an array storing multiple sub-objects with the field url. To search in a sub-field, you need to use the dot-syntax:

BasicDBObject query=new BasicDBObject("Notification.url","www.adf.com");

This will, however, return the whole document with the whole Notification array. You likely only want the sub-document. To filter this, you need to use the two-argument version of Collection.find.

BasicDBObject query=new BasicDBObject("Notification.url","www.example.com");
BasicDBObject fields=new BasicDBObject("Notification.$", 1);

DBCursor f = con.coll.find(query, fields);

The .$ means "only the first entry of this array which is matched by the find-operator"

This should still return one document with a sub-array Notifications, but this array should only contain the entry where url == "www.example.com".

To traverse this document with Java, do this:

BasicDBList notifications = (BasicDBList) f.next().get("Notification"); 
BasicDBObject notification = (BasicDBObject) notifications.get(0);
String url = notification.get("url");

By the way: When your database grows you will likely run into performance problems, unless you create an index to speed up this query:

con.coll.ensureIndex(new BasicDBObject("Notification.url", 1));

edited Jul 26, 2013 at 12:46

answered Jul 26, 2013 at 10:43

Philipp

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9 Comments

Prashant Thorat Over a year ago

When I tried to access "url" seprated by above query String s=(String) f.curr().get("url"); It returns null value

Philipp Over a year ago

@PrashantThorat That's likely because the returned document has no field url. It likely only has a field Notifications which contains a DBList which contains one DBObject which contains a field url. Check it in the debugger.

Philipp Over a year ago

@PrashantThorat f.curr().get("Notifications").get(0).get("url") could work.

Prashant Thorat Over a year ago

Is their any other alternative? I tried it but it gives error at get(0)

Philipp Over a year ago

@PrashantThorat Sorry, but I lost my tarot deck. Please use the debugger and tell me what document f.curr() returns.

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