How do I update MongoDB BSON field value and add a new field in C++?

[johnsam]

I was trying to modify a field in a BSON object, add a new field. I was only able to append the new field but failed to update the existing field. Please advise. Thanks,

const char * ns1 = "mytest.c1";
const char * ns2 = "mytest.c2";

// test insert
conn.insert( ns1 ,BSON( "name" << "eliot" << "num" << 1 ) );
conn.insert( ns1 ,BSON( "name" << "sara" << "num" << 2 ) );
{
    auto_ptr<DBClientCursor> cursor = conn.query( ns1 , BSON( "num" << 1 ) );
    int count = 0;
    while ( cursor->more() ) {
        count++;
        BSONObj obj = cursor->next();
        BSONElement e = obj.getField("name");
        string v ("john");
        e.Val(v);
        BSONObjBuilder b;
        b.appendElements(obj);
        b.append("room", 123);
        conn.insert( ns2, b.obj()  );
    }
}

mongos> db.c1.find()
{ "_id" : ObjectId("5356d3e08b391dea54e2d3f4"), "name" : "eliot", "num" : 1 }
{ "_id" : ObjectId("5356d3e18b391dea54e2d3f5"), "name" : "sara", "num" : 2 }
mongos> db.c2.find()
{ "_id" : ObjectId("5356d3e08b391dea54e2d3f4"), "name" : "eliot", "num" : 1, "room" : 123 }

[Asya Kamsky]

Normally the way to update a field is via an update statement, not by reading it, doing something to it and then saving it (as it's not atomic).

It sort of looks like you aren't really updating as you are saving it to another collection?   In any case, check out update and $set operator for updating fields (existing or not).   There are more examples on this page.

Asya

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