Match the field in mongodb - mongodb

Question

I have mongodb sample data result like this:
{
"_id" : {
"month" : 3,
"day" : 24,
"year" : 2017
},
"commodity" : [
{
"commodityId" : ObjectId("58d434c30da1364f1e2d682d"),
"commodityStock" : "88889s"
}
],
"totalStock" : 0,
"count" : 1.0 }
my question is, How can i get the result where month = 3 with $match?
below is my query:
db.orders.aggregate(
[
{ $match : {_id.month : 3}},
{
$group : {
_id : { month: { $month: "$createdAt" }, day: { $dayOfMonth: "$createdAt" }, year: { $year: "$createdAt" } },
commodity : {$push : {
'commodityId' : "$commodity",
'commodityStock' : "$stock",
}
},
totalStock: { $sum: "$stock" },
count: { $sum: 1 }
}
}
]
)

Answer 1

You could use a $redact pipeline which incorporates the functionality of $project and $match so that you can filter the documents in the collection by using a logical condition with the $cond operator and uses the special operations $$KEEP to "keep" the document where the logical condition is true or $$PRUNE to "remove" the document where the condition was false.
db.orders.aggregate([
{
"$redact": {
"$cond": [
{ "$eq": [{ "$month": "$createdAt" }, 3]},
"$$KEEP",
"$$PRUNE"
]
}
},
{
"$group": {
"_id": {
"month": { "$month": "$createdAt" },
"day": { "$dayOfMonth": "$createdAt" },
"year": { "$year": "$createdAt" }
},
"commodity": {
"$push": {
"commodityId": "$commodity",
"commodityStock": "$stock",
}
},
"totalStock": { "$sum": "$stock" },
"count": { "$sum": 1 }
}
}
])
Keep in mind that $redact does not use indexes, it performs a collection scan, but if you need to take advantage of indexes use the $project and $match pipeline stages as:
db.orders.aggregate([
{
"$project": {
"createdAt": 1,
"month": { "$month": "$createdAt" },
"day": { "$dayOfMonth": "$createdAt" },
"year": { "$year": "$createdAt" },
"commodity": 1,
"stock": 1
}
},
{ "$match": { "month": 3 } },
{
"$group": {
"_id": { "month": "$month", "day": "$day", "year": "$year" },
"commodity": {
"$push": {
"commodityId": "$commodity",
"commodityStock": "$stock",
}
},
"totalStock": { "$sum": "$stock" },
"count": { "$sum": 1 }
}
}
])