MongoDB: add field with conditional number of elements - mongodb

Question

I have next simplified collection
[
{
"key": 1,
"array": [
{ "check": true },
{ "check": false },
{ "check": true }
]
},
{
"key": 2
}
]
I want to add field "count" with number of elements of array with "check"=true, so I expect next result
{
"key": 1,
"array": [
{ "check": true },
{ "check": false },
{ "check": true }
],
"count":2,
},
{
"key": 2,
"count": 0
}
]
I have next query ( it is aggregation, because actually it one of stages of pipeline)
db.collection.aggregate([
{
"$addFields": {
"count": {
"$sum": {
"$cond": {
"if": {
"$eq": ["$array.check",true],
},
"then": 1,
"else": 0,
}
}
},
}
}
])
But I always get count=0.
Can you help me to find error in my query?
Here mongo playground

Answer 1

Instead of using $sum, you can use $filter to filter only the array with "check"=true, then check the size of the resulting array using $size.
db.collection.aggregate([
{
"$addFields": {
"count": {
"$size": {
"$filter": {
"input": { "$ifNull": ["$array", []] }, // default empty array if array is does not exist
"cond": "$$this.check" // only keep the truthy check value
}
}
}
}
}
])
Mongo Playground
Alternatively, if you want to use $sum, you could also map the array to an array of 0 and 1 according to the check value, using $map
db.collection.aggregate([
{
"$addFields": {
"count": {
"$sum": {
"$map": {
"input": { "$ifNull": ["$array", []] },
"in": {
"$cond": ["$$this.check", 1, 0]
}
}
}
}
}
}
])
Mongo Playground

Answer 2

Here is how to achieve this using $reduce
db.collection.aggregate([
{
"$addFields": {
"count": {
"$sum": {
$reduce: {
input: "$array",
initialValue: 0,
in: {
$sum: [
"$$value",
{
$cond: [
"$$this.check",
1,
0
]
}
]
}
}
}
}
}
}
])
Mongo Playground

Answer 3

Using $reduce aggregation array expression operator,
db.test.aggregate( [
{
$addFields: {
count: {
$reduce: {
input: { $ifNull: [ "$array", [] ] },
initialValue: 0,
in: {
$cond: [ { $eq: [ "$$this.check", true ] },
{ $add: [ "$$value", 1 ] },
"$$value"
]
}
}
}
}
}
] )
gets a result as follows:
{
"_id" : ObjectId("5f0c7b691c7c98bb49fd2b50"),
"key" : 1,
"array" : [
{
"check" : true
},
{
"check" : false
},
{
"check" : true
}
],
"count" : 2
}
{ "_id" : ObjectId("5f0c7b691c7c98bb49fd2b51"), "key" : 2, "count" : 0 }