Aggregate inside document in mongodb - mongodb

Question

I have document sales
db.sale.findOne({_id : ObjectId("52ea4dd29dbc7923107ddb97")})
{
"_id" : ObjectId("52ea4dd29dbc7923107ddb97"),
"firm" : ObjectId("52e56c009dbc794999ea5c3d"),
"patient" : {
"last" : "",
"first" : ""
},
"doc" : "",
"hospital" : "",
"discount" : 0,
"dd" : "",
"mode" : "cash",
"invoice" : "300114-undefined-1",
"items" : [
{
"bat" : "BJFE",
"narco" : 0,
"name" : "GDRNCCD",
"mrp" : 1,
"free" : 0,
"qty" : 1,
"item_discount" : 0,
"wpr" : 1,
"exp" : "1425168000000"
},
{
"bat" : "",
"narco" : 0,
"name" : "GDRN vbhjdsfb",
"mrp" : 1,
"free" : 0,
"qty" : 1,
"item_discount" : 0,
"wpr" : 0,
"exp" : "[object Object]"
},
{
"bat" : "",
"narco" : 0,
"name" : "GDRN vbhjdsfb",
"mrp" : 1,
"free" : 0,
"qty" : 1,
"item_discount" : 0,
"wpr" : 0,
"exp" : "[object Object]"
}
],
"date" : ISODate("2014-01-30T00:00:00Z"),
"mob" : "",
"email" : ""
}
How can I Aggregate total numbers if items in one field and sum up mrp *qty of all the items into one field.
I have read the mognodb aggregation but it only aggregates among group of matched documents not inside a single document. Is it possile?
{
"_id" : ObjectId("52ea4dd29dbc7923107ddb97"),
"firm" : ObjectId("52e56c009dbc794999ea5c3d"),
'total_items' : items.length,
"total" : mrp*qty of all items,
}

Answer 1

db.sales.aggregate(
{$unwind: "$items"},
{$project: {_id: 1,firm:1, total: {$multiply: ["$items.mrp", "$items.qty"]}}},
{$group: {_id: {"id":"$_id", firm:"$firm"}, count: {$sum:1} , total : {$sum:"$total"}}}
)
With a slight change : _id contains id and firm, you will need an extra projection to match your desired document, but I don't think it's important.
Plus, you can easily change to group by farm only

Answer 2

Thanks to Orid,I tried this,
db.sale.aggregate(
{ $match :{ firm :ObjectId("52e56c009dbc794999ea5c3d") } },
{$project : {day : {$dayOfMonth : '$date'},items :1,patient :1,invoice :1}},
{$match : {day: {$gte : new Date().getDate()}}},
{$unwind : "$items"},
{$project: {_id: 1,patient:1,invoice :1, total: {$multiply: ["$items.mrp", "$items.qty"]}}},
{$group: {_id: {"id":"$_id", invoice:"$invoice",patient :"$patient"}, count: {$sum:1} , total : {$sum:"$total"}}})
Result
{
"result" : [
{
"_id" : {
"id" : ObjectId("52eab6129dbc7923107ddbaf"),
"invoice" : "310114-undefined-1",
"patient" : {
"last" : "",
"first" : ""
}
},
"count" : 2,
"total" : 25
},
{
"_id" : {
"id" : ObjectId("52eab6129dbc7923107ddbb0"),
"invoice" : "310114-undefined-1",
"patient" : {
"last" : "",
"first" : ""
}
},
"count" : 1,
"total" : 1
},
{
"_id" : {
"id" : ObjectId("52eab6129dbc7923107ddbae"),
"invoice" : "310114-undefined-1",
"patient" : {
"last" : "",
"first" : ""
}
},
"count" : 2,
"total" : 5
}
],
"ok" : 1
}