Processing array of generic BSON documents with MongoDB C++ driver

Question

I have the following document in my MongoDB test database:

> db.a.find().pretty()
{
    "_id" : ObjectId("5113d680732fb764c4464fdf"),
    "x" : [
            {
                "a" : 1,
                "b" : 2
            },
            {
                "a" : 3,
                "b" : 4
            }
          ]
}

I'm trying to access and process the elements in the "x" array. However, it seems that the Mongo driver is identifying it not as an array of JSON document, but as Date type, as shown in the following code:

auto_ptr<DBClientCursor> cursor = c.query("test.a", BSONObj());
while (cursor->more()) {      
  BSONObj r = cursor->next();
  cout << r.toString() << std::endl;
}

which output is:

{ _id: ObjectId('51138456732fb764c4464fde'), x: new Date(1360233558334) }

I'm trying to follow the documentation in http://api.mongodb.org/cplusplus and http://docs.mongodb.org/ecosystem/drivers/cpp-bson-array-examples/, but it is quite poor. I have found other examples of processing arrays, but always with simple types (e.g. array of integer), but not when the elements in the array are BSON documents themselves.

Do you have some code example of procesing arrays which elements are generic BSON elements, please?

Moreover, the ObjectID that mongo shell and the trace shows seem not being the same. Quite weird... — fgalan
– fgalan, Commented Feb 8, 2013 at 8:09

Kay · Accepted Answer · 2013-02-11 23:30:31Z

3

you could use the .Array() method or the getFieldDotted() method: as in the following:

Query query = Query();
auto_ptr<DBClientCursor> cursor = myConn.query("test.a", query);

while( cursor->more() ) {
    BSONObj nextObject = cursor->next();

    cout << nextObject["x"].Array()[0]["a"] << endl;
    cout << nextObject.getFieldDotted("x.0.a") << endl;


}

answered Feb 11, 2013 at 23:30

Kay

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fgalan Over a year ago

I finally found a solution, but thank you anyway for the hint! I didn't know about the [ ] notation in this case and the getFieldDotted() method.

fgalan · Accepted Answer · 2013-02-12 14:28:38Z

At the end, it seems that embeddedObject() method was the key:

auto_ptr<DBClientCursor> cursor = c.query("test.a", BSONObj());
while (cursor->more()) { 
  BSONObj r = cursor->next();
  cout << "Processing JSON document: " << r.toString() << std::endl;
  std::vector<BSONElement> be = r.getField("x").Array();
  for (unsigned int i = 0; i<be.size(); i++) {
      cout << "Processing array element: " << be[i].toString() << std::endl;
      cout << "                 of type: " << be[i].type() << std::endl;
      BSONObj bo = be[i].embeddedObject();
      cout << "Processing a field: " << bo.getField("a").toString() << std::endl;
      cout << "Processing b field: " << bo.getField("b").toString() << std::endl;
  }
 }

I was wrongly retrieving a different ObjectID and a different type (Date instead of array) becuase I was looking to a different collection :$

Sorry for the noise. I hope that the fragment above helps others to figure out how to manipulate arrays using the MongoDB C++ driver.

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