"Structured" grouping query in MongoDB - mongodb

Question

I have the following items collection :
[{
"_id": 1,
"manufactureId": 1,
"itemTypeId": "Type1"
},
{
"_id": 2,
"manufactureId": 1,
"itemTypeId": "Type2"
},
{
"_id": 3,
"manufactureId": 2,
"itemTypeId": "Type1"
}]
I would like to create a query that will return the amount of items for each item type that each manufacturer have in the following structure (or something similar) :
[
{
_id:1, //this would be the manufactureId
itemsCount:{
"Type1":1, //Type1 items count
"Type2":1 //...
}
},
{
_id:2,
itemsCount:{
"Type1":1
}
}
]
I have tried to use the aggregation framework but i couldn't figure out if there is a way to create a "structured" groupby queries with it.
I can easily achieve the desired result by post-processing this simple aggregation query result :
db.items.aggregate([{$group:{_id:{itemTypeId:"$itemTypeId",manufactureId:"$manufactureId"},count:{$sum:1}}}])
but if possible I prefer not to post-process the result.

Answer 1

Data stays data
I would rather use this query which, I believe, will give you the closest data structure to what you want, without post-processing.
Query
db.items.aggregate(
{
$group:
{
_id:
{
itemTypeId: "$itemTypeId",
manufactureId: "$manufactureId"
},
count:
{
$sum: 1
}
},
},
{
$group:
{
_id: "$_id.manufactureId",
itemCounts:
{
"$push":
{
itemTypeId: "$_id.itemTypeId",
count: "$count"
}
}
}
})
Output
{
"_id" : 1,
"itemCounts" : [
{
"itemTypeId" : "Type1",
"count" : 1
},
{
"itemTypeId" : "Type2",
"count" : 1
}
]
},
{
"_id" : 2,
"itemCounts" : [
{
"itemTypeId" : "Type1",
"count" : 1
}
]
}
Data transformed to object fields
This is actually an approach that I wouldn't advice in general. It is harder to manage in your application, because the field names between different objects will be inconsistent and you won't know what object fields to expect in advance. This would be a crucial point if you use a strongly typed language—automatic data binding to your domain objects will become impossible.
Anyway, the only way to get the exact data structure you want is to apply post-processing.
Query
db.items.aggregate(
{
$group:
{
_id:
{
itemTypeId: "$itemTypeId",
manufactureId: "$manufactureId"
},
count:
{
$sum: 1
}
},
},
{
$group:
{
_id: "$_id.manufactureId",
itemCounts:
{
"$push":
{
itemTypeId: "$_id.itemTypeId",
count: "$count"
}
}
}
}).forEach(function(doc) {
var obj = {
_id: doc._id,
itemCounts: {}
};
doc.itemCounts.forEach(function(typeCount) {
obj.itemCounts[typeCount.itemTypeId] = typeCount.count;
});
printjson(obj);
})
Output
{ "_id" : 1, "itemCounts" : { "Type1" : 1, "Type2" : 1 } }
{ "_id" : 2, "itemCounts" : { "Type1" : 1 } }