How can I get a single item from the array and display it as an object? and not as an array Mongodb - mongodb

Question

I have a collection from which I need specific obj e.g. notes.blok2 and notes.curse5 as an object, not as an array
{
"year":2020,
"grade":4,
"seccion":"A",
"id": 100,
"name": "pedro",
"notes":[{"curse":5,
"block":1,
"score":{ "a1": 5,"a2": 10, "a3": 15}
},{"curse":5,
"block":2,
"score":{ "b1": 10,"b2": 20, "b3": 30}
}
]
}
My query
notas.find({
"$and":[{"grade":1},{"seccion":"A"},{"year":2020}]},
{"projection":{ "grade":1, "seccion":1,"name":1,"id":1,
"notes":{"$elemMatch":{"block":2,"curse":5}},"notes.score":1} })
It works but returns notes like array
{
"_id": "55",
"id": 100,
"grade": 5,
"name": "pedro",
"seccion": "A",
"notes": [
{"score": { "b1": 10,"b2": 20, "b3": 30} }
]
}
But I NEED LIKE THIS: score at the same level as others and if doesn't exist show empty "score":{}
{
"year":2020,
"grade":5,
"seccion":"A",
"id": 100,
"name": "pedro",
"score":{ "b1": 10,"b2": 20, "b3": 30}
}

Answer 1

Demo - https://mongoplayground.net/p/XlJqR2DYW1X
You can use aggregation query
db.collection.aggregate([
{
$match: { // filter
"grade": 1,
"seccion": "A",
"year": 2020,
"notes": {
"$elemMatch": {
"block": 2,
"curse": 5
}
}
}
},
{ $unwind: "$notes" }, //break into individual documents
{
$match: { // match query on individual note
"notes.block": 2,
"notes.curse": 5
}
},
{
$project: { // projection
"grade": 1,
"seccion": 1,
"name": 1,
"id": 1,
"score": "$notes.score"
}
}
])
Update
Demo - https://mongoplayground.net/p/mq5Kue3UG42
Use $filter
db.collection.aggregate([
{
$match: {
"grade": 1,
"seccion": "A",
"year": 2020
}
},
{
$set: {
"score": {
"$filter": {
"input": "$notes",
"as": "note",
"cond": {
$and: [
{
$eq: [ "$$note.block",3]
},
{
$eq: [ "$$note.curse", 5 ]
}
]
}
}
}
}
},
{
$project: {
// projection
"grade": 1,
"seccion": 1,
"name": 1,
"id": 1,
"score": {
"$first": "$score.score"
}
}
}
])
If you want empty object for score when match not found you can do -
Demo - https://mongoplayground.net/p/dumax58kgrc
{
$set: {
score: {
$cond: [
{ $size: "$score" }, // check array length
{ $first: "$score" }, // true - take 1st
{ score: {} } // false - set empty object
]
}
}
},