Avoid empty array elements in mongo db - arrays

Question

How to avoid empty array while filtering results while querying a collection in MongoDb
[
{
"_id": ObjectId("5d429786bd7b5f4ae4a64790"),
"extensions": {
"outcome": "success",
"docType": "ABC",
"Roll No": "1"
},
"data": [
{
"Page1": [
{
"heading": "LIST",
"content": [
{
"text": "<b>12345</b>"
},
],
}
],
"highlights": [
{
"name": "ABCD",
"text": "EFGH",
}
],
"marks": [
{
"revision": "revision 1",
"Score": [
{
"maths": "100",
"science": "40",
"history": "90"
},
{
"lab1": "25",
"lab2": "25"
}
],
"Result": "Pass"
},
{
"revision": "revision 1",
"Score": [
{
"maths": "100",
"science": "40"
},
{
"lab1": "25",
"lab2": "25"
}
],
"Result": "Pass"
}
]
}
]
}
]
I am looking for results that has only "history" marks in the score array.
I tried the following query (in mongo 3.6.10) but it returns empty score array as well the array that has history as well
db.getCollection('student_scores').find({
"data.marks.score.history": {
$not: {
$type: 10
},
$exists: true
}
},
{
"extensions.rollNo": 1,
"data.marks.score.history": 1
})
Desired output is
{
"extensions": {
"rollNo": "1"
},
"data": [
{
"marks": [
{
"Score": [
{
"history": "90"
}
]
}
]
}
]
}

Answer 1

I used something like the following;
db.getCollection('student_scores').aggregate([
{
$unwind: "$data"
},
{
$unwind: "$data.marks"
},
{
$unwind: "$data.marks.Score"
},
{
$match: {
"data.marks.Score.history": {
$exists: true,
$not: {
$type: 10
}
}
}
},
{
$project: {
"extensions.Roll No": 1,
"data.marks.Score.history": 1
}
},
{
$group: {
_id: "$extensions.Roll No",
history_grades: {
$push: "$data.marks.Score.history"
}
}
}
])
where I got the following result with your input (I think more readable than your expected output);
[
{
"_id": "1",
"history_grades": [
"90"
]
}
]
where _id represents "extensions.Roll No" value for any given data set.
What do you think?
check with a bigger input on mongoplayground

Answer 2

OK, so I still think the data design here with the Score array is a little off but here is solution that will ensure that a Score array contains only 1 entry and that entry is for a key of history. We use dotpath array diving as a trick to get to the value of history.
c = db.foo.aggregate([
{$unwind: "$data"}
,{$unwind: "$data.marks"}
,{$project: {
result: {$cond: [
{$and: [ // if
{$eq: [1, {$size: "$data.marks.Score"}]}, // Only 1 item...
// A little trick! $data.marks.Score.history will resolve to an *array*
// of the values associated with each object in $data.marks.Score (the parent
// array) having a key of history. BUT: As it resolves, if there is no
// field for that key, nothing is added to resolution vector -- not even a null.
// This means the resolved array could
// be **shorter** than the input. FOr example:
// > db.foo.insert({"x":[ {b:2}, {a:3,b:4}, {b:7}, {a:99} ]});
// WriteResult({ "nInserted" : 1 })
// > db.foo.aggregate([ {$project: {z: "$x.b", n: {$size: "$x.b"}} } ]);
// { "z" : [ 2, 4, 7 ], "n" : 3 }
// > db.foo.aggregate([ {$project: {z: "$x.a", n: {$size: "$x.a"}} } ]);
// { "z" : [ 3, 99 ], "n" : 2 }
//
// You must be careful about this.
// But we also know this resolved vector is of size 1 (see above) so we can go ahead and grab
// the 0th item and that becomes our output.
// Note that if we did not have the requirement of ONLY history, then we would not
// need the fancy $cond thing.
{$arrayElemAt: ["$data.marks.Score.history",0]}
]},
{$arrayElemAt: ["$data.marks.Score.history",0]}, // then (use value of history)
null ] } // else set null
,extensions: "$extensions" // just carry over extensions
}}
,{$match: {"result": {$ne: null} }} // only take good ones.