Develop Reference

How to get object value with dynamic key in $project in mongoDB

I want to retrieve a value from an object with a dynamic key
[
{
"_id": 1,
"item": "sweatshirt",
"price": {
"INR": 45.99
},
"currency": 'INR'
}
]
db.collection.aggregate([
{
"$project": {
"pricenew": "$price.currency"
}
}
])
If I do price.INR it will work fine but here I have currency dynamic, so I want something like price.currency but here currency is coming like "INR" and it gives no data.
I really appreciate any help you can provide.

You need to convert the price object to an array using $objectToArray, filter it and then convert it back, like so:
db.collection.aggregate([
{
$replaceRoot: {
newRoot: {
"$mergeObjects": [
{
"$arrayToObject": {
$map: {
input: {
$filter: {
input: {
"$objectToArray": "$price"
},
cond: {
$eq: [
"$$this.k",
"$currency"
]
}
}
},
in: {
k: "pricenew",
v: "$$this.v"
}
}
}
},
{
_id: "$_id"
}
]
}
}
}
])
Mongo Playground

getting error unknown group operator '$group' in mongoDB

I am trying to count distinct(not unique) or Emp No in same department.but getting error
query failed: unknown group operator '$group'
here is my code
https://mongoplayground.net/p/UvYF9NB7vZx
db.collection.aggregate([
{
$group: {
_id: "$Department",
total: {
"$group": {
_id: "$Emp No"
}
}
}
}
])
Expected output
[
{
"_id": "HUAWEI”,
“total”:1
},
{
"_id": "THBS”,
“total”:2
}
]
THBShave two different Emp No A10088P2C and A20088P2C
HUAWEI have only one Emp No A1016OBW

so, $group is Pipeline stage, you can only use it in upper level.
But for your required output there is lots of ways i believe,
we can do something like this as well:
db.collection.aggregate([
{
$group: {
_id: {
dept: "$Department",
emp: "$Emp No"
},
total: {
"$sum": 1
}
}
},
{
$group: {
_id: "$_id.dept",
total: {
"$sum": 1
}
}
}
])
Here, in first stage we are grouping with Department and its Emp No , and also we are having count of how many Emp No is in each dept.
[this count you can remove though as we are not using it.]
result of this stage will be:
[
{
"_id": {
"dept": "THBS",
"emp": "A10088P2C"
},
"total": 2
},
{
"_id": {
"dept": "THBS",
"emp": "A20088P2C"
},
"total": 1
},
{
"_id": {
"dept": "HUAWEI",
"emp": "A1016OBW"
},
"total": 3
}
]
next on top of this part data, i'm grouping again, with the dept. which comes in $_id.dept, and making count in the same way, which gives the result in your required format.
[
{
"_id": "HUAWEI",
"total": 1
},
{
"_id": "THBS",
"total": 2
}
]
Demo

How can I count the documents in an array depending on their value in mongodb?

I need to count the number of parking spaces _id: 5d752c544f4f1c0f1c93eb23, which has the false value in the excluded property, how can I mount the query?
So far, I've been able to select the number of parking spaces with the excluded: false property. But are selecting from all parking lots
Note that there are two documents symbolizing a parking lot, where each has an array, called parkingSpaces, to record parking space documents.
The first document has 2 vacancies not excluded, so with the property excluded: false and the second has only one vacancy, which is not excluded either.
{
"_id": "5d752c544f4f1c0f1c93eb23",
"name": "estacionamento um",
"parkingSpace": [
{
"_id": "5d752cf54f4f1c0f1c93eb26",
"name": "vg001",
"excluded": true
},
{
"_id": "5d752cf54f4f1c0f1c93eb27",
"name": "vg001",
"excluded": false
},
{
"_id": "5d75339bc411423a9c14ac52",
"name": "vg002",
"excluded": false
}
]
},
{
"_id": "5d7706b60d354b72388a38f4",
"name": "estacionamento dois",
"parkingSpace": [
{
"_id": "5d77078a5173bb63bc87b7ca",
"name": "vg004",
"excluded": false
}
]
}
I need to add the number of parking spaces _id: 5d752c544f4f1c0f1c93eb23, which has the value false in the excluded property.
In the end, I need to return the value 2, referring to _id: 5d752c544f4f1c0f1c93eb23 parking spaces, which have the value false in the excluded property.
So far, with the following query, I was able to select the vacancies with the excluded property with the false value, but it is selecting from all parking lots.
const registeredParkingSpaces = await Parking.aggregate([
{ $unwind: '$parkingSpace' },
{ $match: { 'parkingSpace.excluded': false } },
{
$group: {
_id: parking_id,
total: { $sum: 1 }
}
}
]);
returns:
{
"message": [
{
"_id": "5d752c544f4f1c0f1c93eb23",
"total": 3
}
]
}
But it needs to return:
{
"message": [
{
"_id": "5d752c544f4f1c0f1c93eb23",
"total": 2
}
]
}

In aggregation 1st you match with id then go for the next step you will get your desire count because at this moment you are considering entire document "parkingSpace". Below is the sample code I guess it will work for you
const ObjectId = require('mongoose').Types.ObjectId;
const registeredParkingSpaces = await Parking.aggregate([
{ $match: { _id: objectId(body.id) } }
{ $unwind: '$parkingSpace' },
{ $match: { 'parkingSpace.excluded': false } },
{
$group: {
_id: parking_id,
total: { $sum: 1 }
}
}
]);

This is the query through which you can get the count of total parking .
** UPDATED **
db.collection.aggregate([
{
$match: {
"parkingSpace.excluded": false,
}
},
{
$project: {
parkingSpace: 1
}
},
{
"$group": {
"_id": "$_id",
"count": {
"$sum": {
"$size": {
"$filter": {
"input": "$parkingSpace",
"as": "el",
"cond": {
"$eq": [
"$$el.excluded",
false
]
}
}
}
}
}
}
}
])
You can check the solution from this LINK
For more information about $sum : Visit the official official document
Through this you get your solution .

mongodb to return object from facet

Is it possible to have facet to return as an object instead of an array? It seems a bit counter intuitive to need to access result[0].total instead of just result.total
code (using mongoose):
Model
.aggregate()
.match({
"name": { "$regex": name },
"user_id": ObjectId(req.session.user.id),
"_id": { "$nin": except }
})
.facet({
"results": [
{ "$skip": start },
{ "$limit": finish },
{
"$project": {
"map_levels": 0,
"template": 0
}
}
],
"total": [
{ "$count": "total" },
]
})
.exec()

Each field you get using $facet represents separate aggregation pipeline and that's why you always get an array. You can use $addFields to overwrite existing total with single element. To get that first item you can use $arrayElemAt
Model
.aggregate()
.match({
"name": { "$regex": name },
"user_id": ObjectId(req.session.user.id),
"_id": { "$nin": except }
})
.facet({
"results": [
{ "$skip": start },
{ "$limit": finish },
{
"$project": {
"map_levels": 0,
"template": 0
}
}
],
"total": [
{ "$count": "total" },
]
})
.addFields({
"total": {
$arrayElemAt: [ "$total", 0 ]
}
})
.exec()

You can try this as well
Model
.aggregate()
.match({
"name": { "$regex": name },
"user_id": ObjectId(req.session.user.id),
"_id": { "$nin": except }
})
.facet({
"results": [
{ "$skip": start },
{ "$limit": finish },
{
"$project": {
"map_levels": 0,
"template": 0
}
}
],
"total": [
{ "$count": "total" },
]
})
.addFields({
"total": {
"$ifNull": [{ "$arrayElemAt": [ "$total.total", 0 ] }, 0]
}
})
.exec()

imagine that you want to pass the result of $facet to the next stage, let's say $match. well $match accepts an array of documents as input and return an array of documents that matched an expression, if the output of $facet was just an element we can't pass its output to $match because the type of output of $facet is not the same as the type of input of $match ($match is just an example). In my opinion it's better to keep the output of $facet as array to avoid handling those types of situations.
PS : nothing official in what i said

Mongodb find() only include non-empty arrays

What I'm trying to achieve with a find query is to only include "someArray"s if it's inner array is not empty. For example the JSON below:
{
"document": "some document",
"someArray": [
{
"innerArray": [
"not empty"
]
},
{
"innerArray": [
[] //empty
]
}
]
}
Would return this:
{
"document": "some document",
"someArray": [
{
"innerArray": [
"not empty"
]
}
]
}
I'm using the following find:
Visit.find({'someArray.innerArray.0': {$exists: true}}, function(err, data){});
However, this returns all data.
Have also tried:
Visit.find({}, {'someArray.innerArray': {$gt: 0}}, function(err, data) {});
But this returns nothing
Any ideas on how to approach this?
Cheers

The general case here to check for a non-empty array is to check to see if the "first" element actually exists. For single matches you can project with the positional $ operator:
Vist.find(
{ "someArray.innerArray.0": { "$exists": true } },
{ "document": 1,"someArray.$": 1},
function(err,data) {
}
);
If you need more than a single match or have arrays nested more deeply than this, then the aggregation framework is what you need to handle the harder projection and/or "filter" the array results for more than one match:
Visit.aggregate(
[
// Match documents that "contain" the match
{ "$match": {
"someArray.innerArray.0": { "$exists": true }
}},
// Unwind the array documents
{ "$unwind": "$someArray" },
// Match the array documents
{ "$match": {
"someArray.innerArray.0": { "$exists": true }
}},
// Group back to form
{ "$group": {
"_id": "$_id",
"document": { "$first": "$document" },
"someArray": { "$push": "$someArray" }
}}
],function(err,data) {
}
)
Worth noting here that you are calling this "empty" but in fact is is not, as it actually contains another empty array. You probably don't want to do that with real data, but if you have then you would need to filter like this:
Visit.aggregate(
[
{ "$match": {
"someArray": { "$elemMatch": { "innerArray.0": { "$ne": [] } } }
}},
{ "$unwind": "$someArray" },
{ "$match": {
"someArray.innerArray.0": { "$ne": [] }
}},
{ "$group": {
"_id": "$_id",
"document": { "$first": "$document" },
"someArray": { "$push": "$someArray" }
}}
],function(err,data) {
}
);