modify array element's value with Java MongoDB driver - mongodb

Question

I want to modify array element using Java MongoDB driver. I am able to insert new pair to the array, but can't modify value corresponding to particular key.
How can I increase a by 2 in dummy array for document
{ "_id" : ObjectId("57a87614d03a435e4be44bb9"), "dummy" : [ { "a" : 1 }, { "b" : 5 } ] }
using Java MongoDB driver?
Here is what I've tried
BasicDBObject query = new BasicDBObject();
query.put("_id",doc_id_here);
BasicDBObject incValue = new BasicDBObject("dummy.$.a", 1);
BasicDBObject intModifier = new BasicDBObject("$inc", incValue);
coll.update(query, intModifier, false, false, WriteConcern.SAFE);

Answer 1

Your query in mongo shell
db.collection.update(
{ "_id": ObjectId("57a87614d03a435e4be44bb9") },
{ $inc: { "dummy.$.a": 1 } }
);
will result in error
The positional operator did not find the match needed from the query.
Unexpanded update: dummy.$.a
because in order to use positional $ operator for dummy array you need to set condition on this array in your query
db.collection.update(
{ "_id": ObjectId("57a87614d03a435e4be44bb9"), "dummy.a": { $exists: true } },
{ $inc: { "dummy.$.a": 1 } }
);
and then it will increment a as you expect.
With Java MongoDB driver it will be
BasicDBObject query = new BasicDBObject();
query.put("_id", new ObjectId("57a87614d03a435e4be44bb9"));
query.put("dummy.a", new BasicDBObject("$exists", true));
BasicDBObject incValue = new BasicDBObject("dummy.$.a", 1);
BasicDBObject intModifier = new BasicDBObject("$inc", incValue);
coll.update(query, intModifier, false, false, WriteConcern.SAFE);
Advice: before trying to construct your query with Java MongoDB Driver first try if it works in mongo shell.