MongoDB $group with nested object key $sum

Question

The object structure is as follows:

{
    "_id" : ObjectId("53434bfe234416601c8b4567"),
    "package" : {
        "id" : "3",
        "title" : "P3",
        "price" : "8.00",
        "votes" : "8",
        "created_at" : "2014-04-08 00:29:35",
        "updated_at" : "2014-04-08 00:31:39"
    },
    "status" : "success",
    "user_id" : "5341dd9c234416de758b4570",
    "updated_at" : ISODate("2014-04-08T01:08:52.168Z"),
    "created_at" : ISODate("2014-04-08T01:08:14.849Z")
}

What I'm trying to do is collect the sum of package.price with this:

db.transactions.aggregate({
    $group:{
        _id: null,
        'total': {$sum: "$package.price"}
     }
})

And the total is always int 0, here's the result:

{
    "result" : [ 
        {
            "_id" : null,
            "total" : 0
        }
    ],

    "ok" : 1
}

Answer 1

For anyone who might fall into this again, as @Anand said, it had to do with the type of the field. Always make sure it is of a "summable" type like int, double, float, etc.