How to group mongo sub document - mongodb

Question

I am newbie to mongo, i am trying to take the group by values in a subdocument, and having the mongo collection structure as like :
{
"_id" : ObjectId("589d4e4b270f8b1635d400b1"),
"myShopId" : 439,
"products" : [
{
"productId" : "1234",
"productName" : "sambarpowder 500 gm",
"productCategory" : "masala",
"mrp" : "90",
"_id" : ObjectId("589d595f6da20b72fe006ea9")
},
{
"productId" : "5678",
"productName" : "moong dhal 200 gms",
"productCategory" : "dhal",
"mrp" : "38 ",
"_id" : ObjectId("589d595f6da20b72fe006eaa")
},
{
"productId" : "5678",
"productName" : "moong dhal 200 gms",
"productCategory" : "dhal",
"mrp" : "38 ",
"_id" : ObjectId("589d595f6da20b72fe006eaa")
}
],
"isAlive" : 1,
"__v" : 3
}
Here, I want to do group by in this.
for eg in mysql:
select productCategory from products where shopId = '439' groupby productCategory
How can i achieve the group by in mongo sub document
My Expected output is like :
category : [{
productCategory : masala
_id : ObjectId("589d595f6da20b72fe006ea9")
},
{
productCategory : dhal
_id : ObjectId("589d595f6da20b72fe006eaa")
}
]

Answer 1

Hope this will help,
db.test.aggregate([{
$match: {
myShopId: 439
}
}, {
$unwind: "$products"
}, {
$group: {
_id: {
"productCategory": "$products.productCategory"
},
"id": {
$first: "$products._id"
}
}
}])
Output:
{ "_id" : { "productCategory" : "dhal" }, "id" : ObjectId("589d595f6da20b72fe006eaa") }
{ "_id" : { "productCategory" : "masala" }, "id" : ObjectId("589d595f6da20b72fe006ea9") }