MongoDB how to select only specific sub_elements for a document? - mongodb

Question

In MongoDB I have documents like this:
{
"_id" : ObjectId("5cc9f3c87aa1024e079a3abf"),
"created_at" : ISODate("2019-04-01T00:00:00.000Z"),
"demographics" : [
{
"key" : "gender",
"value" : "male"
},
{
"key" : "birth_year",
"value" : 1992
},
{
"key" : "city_or_rural",
"value" : "rural"
},
{
"key" : "car_purchase_intention",
"value" : "no"
},
{
"key" : "education_level",
"value" : "high"
},
{
"key" : "age",
"value" : 26
}
]
}
And I would like to make a query by "created_at" but for each document in the result only returning the demographics elements which key are "age" or "gender".
I am trying combinations of $unwind and $project but I can not obtain any proper result.
I am expecting results as this:
# 1
{
"_id" : ObjectId("5cc9f3c87aa1024e079a3abf"),
"created_at" : ISODate("2019-04-01T00:00:00.000Z"),
"demographics" : [
{
"key" : "gender",
"value" : "male"
},
{
"key" : "age",
"value" : 26
}
]
}
# 2
{
"_id" : ObjectId("5cc9f3c87aa1024e079axxx"),
"created_at" : ISODate("2019-04-01T00:00:00.000Z"),
"demographics" : [
{
"key" : "gender",
"value" : "female"
},
{
"key" : "age",
"value" : 56
}
]
}

Answer 1

You can use $filter aggregation after you match by created_at field.
$filter selects a subset of an array to return based on the specified condition and returns an array with only those elements that match the condition.
db.collection.aggregate([
{
$match: {
created_at: ISODate("2019-04-01T00:00:00.000Z")
}
},
{
$project: {
created_at: "$created_at",
demographics: {
$filter: {
input: "$demographics",
as: "item",
cond: {
$in: [
"$$item.key",
[
"gender",
"age"
]
]
}
}
}
}
}
])
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