Grouping by multiple fields in MongoDB

This is my document structure:

{
    "ip_address" : "192.168.133.12",
    "timestamp" : "2014-08-28T06:41:24",
    "response" : 400,
    "uri" : {
        "term" : "Something",
        "page" : "10",
        "category" : "category 10"
    }
}

      

        
        
        
      

    

If I want to do groupby on one "response" field, I will do it like this:

db.collName.aggregate({ $group : {_id : "$response", total : { $sum : 1 }} });

      

        
        
        
      

    

How do I group by 2 or say 3 fields? Can multiple fields be grouped together so that they form a collection with similar values?

I mean something like this:

{
    "result" : [ 
        {
            "_id" : "responseValue" + "ip_addressValue",
            "totaling" : 3
        }
    ],
    "ok" : 1
}