Select sub-documents where field's value is in an some array - mongodb

Question

I want to filter according the sub documents, but actually I am repeating the document for each sub document. I want one document and a list of sub documents if that is the case.
My data looks like:
{
"_id" : ObjectId("582eeb5f75f58055246bd22d"),
"filename" : "file1",
"cod" : NumberLong(90),
"subdocs" : [
{
"length" : NumberLong(10),
"desc" : "000"
},
{
"length" : NumberLong(15),
"desc" : "011"
},
{
"length" : NumberLong(30),
"desc" : "038"
}
]
}
{
"_id" : ObjectId("582eeb5f75f58055246bd22e"),
"filename" : "file2",
"cod" : NumberLong(95),
"subdocs" : [
{
"length" : NumberLong(11),
"desc" : "000"
},
{
"length" : NumberLong(21),
"desc" : "018"
},
{
"length" : NumberLong(41),
"desc" : "008"
}
]
}
I am using this query to filter for the desc (000, 011) on the subdocs
db.ftmp.aggregate(
{ $match:
{ "subdocs.desc":
{ $in: ["000", "011"] }
}
},
{ $unwind : "$subdocs" },
{ $match :
{ "subdocs.desc" :
{ $in:["000", "011"] }
}
}
)
But the result shows 3 documents, 1 document for each sub-document that matches with that query.
{
"_id" : ObjectId("582eeb5f75f58055246bd22d"),
"filename" : "file1",
"cod" : NumberLong(90),
"subdocs" : {
"length" : NumberLong(10),
"desc" : "000"
}
}
{
"_id" : ObjectId("582eeb5f75f58055246bd22d"),
"filename" : "file1",
"cod" : NumberLong(90),
"subdocs" : {
"length" : NumberLong(15),
"desc" : "011"
}
}
{
"_id" : ObjectId("582eeb5f75f58055246bd22e"),
"filename" : "file2",
"cod" : NumberLong(95),
"subdocs" : {
"length" : NumberLong(11),
"desc" : "000"
}
}
However I want to get: file1 with the subdocuments with desc 000 and 011, and file2 with the subdocumnt 000
{
"_id" : ObjectId("582eeb5f75f58055246bd22d"),
"filename" : "file1",
"cod" : NumberLong(90),
"subdocs" : [
{
"length" : NumberLong(10),
"desc" : "000"
},
{
"length" : NumberLong(15),
"desc" : "011"
}
]
}
{
"_id" : ObjectId("582eeb5f75f58055246bd22e"),
"filename" : "file2",
"cod" : NumberLong(95),
"subdocs" : {
"length" : NumberLong(11),
"desc" : "000"
}
}
What is the correct way to do that? Any idea?

Answer 1

First of all using the $unwind operator as mentioned in this answer will cause a drop of performance in your application because unwinding your array result in more documents to process down in the pipeline. There is a better way to achieve this since MongoDB 2.6.
That being said, this is a perfect job for the $filter operator new in MongoDB 3.2.
The most efficient way to do this is in MongoDB 3.4. MongoDB 3.4 introduced the $in array operator for the aggregation framework which can be used in the $filter conditional expression which, when evaluates to true include the sub-document in the resulting array.
let values = [ '000', '011' ];
db.collection.aggregate([
{ "$project": {
"filename": 1,
"cod": 1,
"subdocs": {
"$filter": {
"input": "$subdocs",
"as": "s",
"cond": { "$in": [ "$$s.desc", values ] }
}
}
}}
])
In MongoDB 3.2 we need a slightly different approach because we can use the $in operator there. But luckily we have the $setIsSubset operator and as you might have guess performs a set operation on two array and return true if the first array is a subset of the second array. Because $setIsSubset first expression must be an array, need to make the desc field an array in our pipeline. To do this, we simply use the [] bracket the create that array field which is new MongoDB 3.2
db.collection.aggregate([
{ "$project": {
"filename": 1,
"cod": 1,
"subdocs": {
"$filter": {
"input": "$subdocs",
"as": "s",
"cond": { "$setIsSubset": [ [ "$$s.desc" ], values ] }
}
}
}}
])
MongoDB 3.0 is dead to me but if for some reasons you are running that version, you can use the $literal operator to return the one element array you need for the set operation and the $setDifference operator. This is left as exercise to the reader.

Answer 2

You just need to add $group & $push. First you $unwind the subdocs to apply the $match followed by $group on id and $push the grouped subdocs.
db.ftmp.aggregate({
$unwind: "$subdocs"
}, {
$match: {
"subdocs.desc": {
$in: ["000", "011"]
}
}
}, {
$group: {
_id: "$_id",
subdocs: {
$push: "$subdocs"
},
filename: {
$first: "$filename"
},
cod: {
$first: "$cod"
}
}
})