Count if a value exists inside the array of objects [ MongoDB ] - mongodb

Question

I am developing a simple chat application with MongoDB and got stuck into a situation.
My document in database is as
{
"_id" : ObjectId("605a217ed8168f4c262f4782"),
"message" : "Hi, This is a test message",
"created" : ISODate("2021-03-23T17:12:30.000Z"),
"user" : {
"_id" : ObjectId("5977af7df1d8cc4623283b14"),
"name" : "Sender Of Message"
},
"recipients" : [
{
"_id" : ObjectId("5977af7df1d8cc4623283b14"),
"time" : ISODate("2021-03-23T17:12:30.000Z")
},
{
"_id" : ObjectId("5df50a5eaa0e3c3104006101"),
"time" : ISODate("2021-03-23T17:12:35.000Z")
}
],
"target" : {
"_id" : ObjectId("5df50a5eaa0e3c3104006101"),
"name" : "Target Person"
},
"status" : 1
}
When I try to get the last message with the unread count of the user I am always getting 1
Here is the query that I tried on.
db.collection.aggregate([
{ $match: { 'target._id': ObjectId('5df50a5eaa0e3c3104006101'), status: 1 } },
{ $sort: { _id: -1 } },
{
$group: {
_id: '$user._id',
doc: { $first: '$$ROOT' },
unread: {
$sum: {
$cond: {
if: { $ne: [ ObjectId('5df50a5eaa0e3c3104006101'), '$recipients._id' ] },
then: 1,
else: 0
}
}
}
}
}
])
If the collection contains even just the one document above, it is supposed to give 0 as the object inside the recipients array already contains the _id as ObjectId('5df50a5eaa0e3c3104006101'), but I'm getting 1 for the unread count. Any help?
Here is the output that I get from the query
{
"_id" : ObjectId("5977af7df1d8cc4623283b14"),
"doc" : {
"_id" : ObjectId("605a217ed8168f4c262f4782"),
"message" : "Hi, This is a test message",
"created" : ISODate("2021-03-23T17:12:30.000Z"),
"user" : {
"_id" : ObjectId("5977af7df1d8cc4623283b14"),
"name" : "Sender Of Message"
},
"recipients" : [
{
"_id" : ObjectId("5977af7df1d8cc4623283b14"),
"time" : ISODate("2021-03-23T17:12:30.000Z")
},
{
"_id" : ObjectId("5df50a5eaa0e3c3104006101"),
"time" : ISODate("2021-03-23T17:12:35.000Z")
}
],
"target" : {
"_id" : ObjectId("5df50a5eaa0e3c3104006101"),
"name" : "Target Person"
},
"status" : 1
},
"unread" : 1.0
}
I know why its showing with the count as 1
The array recipients contains an object with _id as ObjectId("5977af7df1d8cc4623283b14") inside it, so its a non matching condition. Which is causing the if condition to be satisfied and produce a value 1.
But I need to figure out how to query it to get the actual value.
Please note that I cant use $push operator on recipients array as it might have greater amount of object ( maybe in future )

Answer 1

Thanks for the support, but I have found the answer by myself.
Here is my approch to get the data as per the requirement.
Instead of searching for the records within the array what I did is
Filtered the data array to the _id that I don't need, so the array will have exactly one document or else it will be empty.
When taking the negation of the condition. ie, when there is one value in the array I need the counter to be 0 or else it should be 1
So I used the $size to check the array's size and $filter to filter out the other _ids and then used $sum to increment the counter as required.
db.collection.aggregate([
{ $match: { 'target._id': ObjectId('5df50a5eaa0e3c3104006101'), status: 1 } },
{ $sort: { _id: -1 } },
{
$group: {
_id: '$user._id',
doc: { $first: '$$ROOT' },
unread: {
$sum: {
$cond:{
if: {
$size: {
$filter: {
input: '$recipients',
as: 'item',
cond: { $eq: [ ObjectId('5df50a5eaa0e3c3104006101'), '$$item._id' ] }
}
}
},
then: 0,
else: 1
}
}
}
}
}
])

Answer 2

Try to Use like this:
db.getCollection('test').aggregate([
{ $match: { 'target._id': ObjectId('5df50a5eaa0e3c3104006101'), status: 1
} },
{ $sort: { _id: -1 } },
{ $unwind: { path: "$recipients", preserveNullAndEmptyArrays: true } },
{
$group: {
_id: '$user._id',
doc: { $first: '$$ROOT' },
unread: {
$sum: {
$cond: {
if: { $ne: ['$recipients._id',
ObjectId('5df50a5eaa0e3c3104006101') ] },
then: 1,
else: 0
}
}
}
}
}
])