MongoDB - How to bring age group data - mongodb

Question

How to bring age group base data from a collection in MongoDB i.e how many people are 0-18, 19-24, 25-34, 35+
[
{
"_id": ObjectId("608be7c608c7de2367c89638"),
"status": true,
"gender": "Male",
"first_name": "Vinter",
"last_name": "R",
"dob": "1-2-1999"
},
{
"_id": ObjectId("608be7c608c7de2267c89639"),
"status": true,
"gender": "Male",
"first_name": "Ray",
"last_name": "Morgan",
"dob": "1-2-2015"
}
....
]
See the Mongo Playground:
https://mongoplayground.net/p/4ydNg9Plh6P

Answer 1

Interesting question!
Would like to credit to #Takis and #YuTing.
Good hint from #Takis's comment on $bucket.
#YuTing's answer is good.
Think this answer is shorter by utilizing the feature provided by MongoDB.
$toDate - Convert date string to Date (supported for version 4.0 above).
$dateDiff - Date subtraction and get the unit (Supported in version 5).
$$CURRENT - Variable to get the current iterated document. For adding into persons array field (via $push).
$switch - To display group value based on conditions (Optional).
db.collection.aggregate([
{
"$addFields": {
"age": {
$dateDiff: {
startDate: {
$toDate: "$dob"
},
endDate: "$$NOW",
unit: "year"
}
}
}
},
{
$bucket: {
groupBy: "$age",
// Field to group by
boundaries: [
0,
19,
25,
35
],
// Boundaries for the buckets
default: "Other",
// Bucket id for documents which do not fall into a bucket
output: {
// Output for each bucket
"count": {
$sum: 1
},
"persons": {
$push: "$$CURRENT"
}
}
}
},
{
$project: {
_id: 0,
group: {
$switch: {
branches: [
{
case: {
$lt: [
"$_id",
19
]
},
then: "0-18"
},
{
case: {
$lt: [
"$_id",
25
]
},
then: "19-24"
},
{
case: {
$lt: [
"$_id",
35
]
},
then: "25-34"
}
],
default: "35+"
}
},
count: 1,
persons: 1
}
}
])
Sample Mongo Playground

Answer 2

use $bucket
db.collection.aggregate([
{
$bucket: {
groupBy: {
"$subtract": [
{
$year: new Date()
},
{
$toInt: {
$substr: [
"$dob",
{
$subtract: [
{
$strLenCP: "$dob"
},
4
]
},
4
]
}
}
]
},
// Field to group by
boundaries: [
0,
19,
25,
35,
100
],
// Boundaries for the buckets
default: "Other",
// Bucket id for documents which do not fall into a bucket
output: {
// Output for each bucket
"count": {
$sum: 1
},
"artists": {
$push: {
"name": {
$concat: [
"$first_name",
" ",
"$last_name"
]
},
"age": {
"$subtract": [
{
$year: new Date()
},
{
$toInt: {
$substr: [
"$dob",
{
$subtract: [
{
$strLenCP: "$dob"
},
4
]
},
4
]
}
}
]
}
}
}
}
}
}
])
mongoplayground