Question

I am working on a existing java based application which is using MongoDB

In one of the class I saw a mongo db group as shown below

BasicDBList result = (BasicDBList) coll.group(key, cond, initial, reduce);

Where key, cond, initial are three different BasicDBObject‘s and reduce is a function in the form of a String

My question is can we see the query that is formed ??

Answer 1

If you have the source code for the MongoDB Java driver, you can look at com.mongodb.GroupCommand to see how this query is built up, it looks something like this:

    BasicDBObject args  = new BasicDBObject();
    args.put( "ns" , input );
    args.put( "key" , keys );
    args.put( "cond" , condition );
    args.put( "$reduce" , reduce );
    args.put( "initial" , initial );
    if ( finalize != null )
        args.put( "finalize" , finalize );
    return new BasicDBObject( "group" , args );

What this translates to is a group command (http://docs.mongodb.org/manual/reference/command/group/) which looks like this:

{ "group" : { "ns" : "coll" , 
              "key" : {<keys>} , 
              "cond" : {<cond>} , 
              "$reduce" : "<reduce>" , 
              "initial" : { }
            }
}