MongoDB project the documents with count greater than 2 [duplicate] - mongodb

Question

This question already has answers here:
Query for documents where array size is greater than 1
(14 answers)
Closed 6 years ago.
I have a collection like
{
"_id": "201503110040020021",
"Line": "1", // several documents may have this Line value
"LineStart": ISODate("2015-03-11T06:49:35.000Z"),
"SSCEXPEND": [{
"Secuence": 10,
"Title": 1,
},
{
"Secuence": 183,
"Title": 613,
},
...
],
} {
"_id": "201503110040020022",
"Line": "1", // several documents may have this Line value
"LineStart": ISODate("2015-03-11T06:49:35.000Z"),
"SSCEXPEND": [{
"Secuence": 10,
"Title": 1,
},
],
}
SSCEXPEND is an array. I am trying to count the size of SSC array and project if the count is greater than or equal to 2. My query is something like this
db.entity.aggregate(
[
{
$project: {
SSCEXPEND_count: {$size: "$SSCEXPEND"}
}
},
{
$match: {
"SSCEXPEND_count2": {$gte: ["$SSCEXPEND_count",2]}
}
}
]
)
I am expecting the output to be only the the first document whose array size is greater than 2.
Project part is working fine and I am able to get the counts but I need to project only those which has count greater than or equal to two but my match part is not working. Can any one guide me as where am I going wrong?

Answer 1

You need to project the other fields and your $match pipeline will just need to do a query on the newly-created field to filter the documents based on the array size. Something like the following should work:
db.entity.aggregate([
{
"$project": {
"Line": 1,
"LineStart": 1, "SSCEXPEND": 1,
"SSCEXPEND_count": { "$size": "$SSCEXPEND" }
}
},
{
"$match": {
"SSCEXPEND_count": { "$gte": 2 }
}
}
])
Sample Output:
/* 0 */
{
"result" : [
{
"_id" : "201503110040020021",
"Line" : "1",
"LineStart" : ISODate("2015-03-11T06:49:35.000Z"),
"SSCEXPEND" : [
{
"Secuence" : 10,
"Title" : 1
},
{
"Secuence" : 183,
"Title" : 613
}
],
"SSCEXPEND_count" : 2
}
],
"ok" : 1
}

Answer 2

This is actually a very simple query, where the trick is to use a property of "dot notation" in order to test the array. All you really need to ask for is documents where the array index of 2 $exists, which means the array must contain 3 elements or more:
db.entity.find({ "SSCEXPEND.2": { "$exists": true } })
It's the fastest way to do it and can even use indexes. No need for calculations in aggregation operations.