Mongodb embedded document - aggregation query - mongodb

Question

I have got the below documents in Mongo database:
db.totaldemands.insert({ "data" : "UKToChina", "demandPerCountry" :
{ "from" : "UK" , to: "China" ,
"demandPerItem" : [ { "item" : "apples" , "demand" : 200 },
{ "item" : "plums" , "demand" : 100 }
] } });
db.totaldemands.insert({ "data" : "UKToSingapore",
"demandPerCountry" : { "from" : "UK" , to: "Singapore" ,
"demandPerItem" : [ { "item" : "apples" , "demand" : 100 },
{ "item" : "plums" , "demand" : 50 }
] } });
I need to write a query to find the count of apples exported from UK to any country.
I have tried the following query:
db.totaldemands.aggregate(
{ $match : { "demandPerCountry.from" : "UK" ,
"demandPerCountry.demandPerItem.item" : "apples" } },
{ $unwind : "$demandPerCountry.demandPerItem" },
{ $group : { "_id" : "$demandPerCountry.demandPerItem.item",
"total" : { $sum : "$demandPerCountry.demandPerItem.demand"
} } }
);
But it gives me the output with both apples and plums like below:
{ "_id" : "apples", "total" : 300 }
{ "_id" : "plums", "total" : 150 }
But, my expected output is:
{ "_id" : "apples", "total" : 300 }
So, How can I modify the above query to return only the count of apples exported from UK ?
Also, is there any other better way to achieve the output without unwinding ?

Answer 1

You can add another $match to get only apples.
As you have embedded document structure and performing aggregation, $unwind is required here. The alternate option could be map and reduce. However, unwind is most suitable here.
If you are thinking about performance, unwind shouldn't cause performance issue.
db.totaldemands.aggregate(
{ $match : { "demandPerCountry.from" : "UK" ,
"demandPerCountry.demandPerItem.item" : "apples" } },
{ $unwind : "$demandPerCountry.demandPerItem" },
{ $group : { "_id" : "$demandPerCountry.demandPerItem.item",
"total" : { $sum : "$demandPerCountry.demandPerItem.demand"
} } },
{$match : {"_id" : "apples"}}
);