Getting subdocument element's count per index inside an array and updating the subdocument key - subdocument in array(IN MONGODB)

Question

How to get subdocument element's count inside an array and how to update the subdocument's key in MongoDB

For eg, following is the whole doc stored in mongodb:

{
    "CompanyCode" : "SNBN",
    "EventCode" : "ET00008352",
    "EventName" : "Sunburn Presents Avicii India Tour",
    "TktDetail" : [ 
        {
            "Type" : "Category I",
            "Qty" : {
                "10-Dec" : {
                    "value" : 58
                },
                "11-Dec" : {
                   "value" : 83
                },
                "12-Dec" : {
                   "value" : 100
                } 
            }
        }, 
        {
            "Type" : "Category II",
            "Qty" : {
                "10-Dec" : {
                   "value" : 4
                },
                "11-Dec" : {
                    "value" : 7
                },
                "12-Dec" : {
                    "value" : 8
                }
            }
        }, 
        {
            "Type" : "PRICE LEVEL 1",
            "Qty" : {
                "11-Dec" : {
                    "value" : 2
                }
            }
        }, 
        {
            "Type" : "CatIV",
            "Qty" : {
                "18-Dec" : {
                    "value" : 20
                }
            }
        }
    ],
    "TransDate" : [ 
        "10-Dec-2013", 
        "11-Dec-2013", 
        "12-Dec-2013", 
    ],
    "VenueCode" : "SNBN",
    "VenueName" : "Sunburn",
    "_id" : ObjectId("52452db273b92012c41ad612")
}

Here TktDetail is an array, inside which there is a Qty subdoc which contains multiple elements, I want to know how to get the elements count inside Qty per index?

For example, the 0th index of TktDetail array contains 1 Qty subdoc, which further has a element count of 3, whereas 3rd index has element count of 1 in Qty subdoc.

If I want to update the subdoc key, like, I want to update the date in Qty from "10-Dec" to "10-Dec-2013", how is it possible?

Thanks in advance, looking for a reply ASAP..

Answer 1

So the first thing here is that you actually asked two questions, being "how do I get a count of the items under Qty?" and "how can I change the names?". Now while normally unrelated I'm going to treat them as the same thing.

What you need to do is change your schema and in doing so I'm going to allow you to get the count of items and I'm going to encourage you to change those field names as well. Specifically you need a schema like this:

"TktDetail" : [ 
    {
        "Type" : "Category I",
        "Qty" : [
            { "date": ISODate("2013-12-10T00:00:00.000Z") , "value" : 58  },
            { "date": ISODate("2013-12-11T00:00:00.000Z"), "value" : 83  },
            { "date": ISODate("2013-12-01T00:00:00.000Z"), "value" : 100 },
        ]
    },

All the gory details are in my answer here to a similar question. But the problem basically is that when you use sub-documents in the way you have you are ruining your chances of doing any meaningful query operations on this, as to get at each element you must specify the full path to get there.

That answer has more detail, but the case is you really want an array. The trade-off, it's a little harder to update, especially considering you have nested arrays, but it's a lot easier to add and much easier to query.

Also, and related, change your dates to be dates and not strings. The strings, are no good for comparisons inside MongoDB. With them set as proper BSON dates (noting I clipped them to the start of day) you can compare, and query ranges and do useful things. Your application code will be happy to as the driver will return a real date object, rather than something you have to manipulate "both ways".

So once you have read through, understood and implemented this, on to counting:

db.collection.aggregate([

    // Unwind the TktDetail array to de-normalize
    {"$unwind": "$TktDetail"},

    // Also Unwind the Qty array
    {"$unwind": "$Qty" },

    // Get some group information and count the entries
    {"$group": { 
        "_id": {
            "_id": "$_id,
            "EventCode": "$EventCode",
            "Type": "$TktDetail.Type"
        },
        "Qty": {"$sum": 1 }
    }},

    // Project nicely
    {"$project": { 
        "_id": 0,
        "EventCode": "$_id.EventCode",
        "Type: "$_id.Type",
        "Qty": 1,
    }},

    // Let's even sort it 
    {"$sort": { "EventCode": 1, "Qty" -1 }}

])

So that allowed us to get a count of the items in Qty for each EventCode by Type with the Qty ordered higest to lowest.

And that is not possible on your current schema without loading and traversing each document in code.

So there is the case. Now if you want to ignore this and just go about changing the sub-document key names, then you'll need to do remove the key and underlying document and replace with the new key name, using update:

db.collection.update(
    { EventCode: "ET00008352"},
    { $unset:{ "TktDetail.0.Qty.10-Dec": "" }}
)

db.collection.update(
    { EventCode: "ET00008352"},
    { $set:{ "TktDetail.0.Qty.10-Dec-2013": { value: 58 } }}
)

And you'll need to do that for every item that you have.

So you either work out that schema conversion or otherwise have a lot of work anyway in order to change the keys. For myself, I'd do it properly, and only do it once so I didn't run into the next problem later.