Aggregate query with document containing array of objects - mongodb

Question

I am facing issue with the aggregation query on MongoDB.
I have a document in following structure:
[{
"_id": ObjectId("19a5070b808028108101"),
"arr_vs": [
{
"arr_id": "one",
"val": 5
},
{
"arr_id": "two",
"val": 5
}]
},
{
"_id": ObjectId("19a5070b80802810810"),
"arr_vs": [
{
"arr_id": "one",
"val": 5
},
{
"arr_id": "two",
"val": 2
},{
"arr_id": "three",
"val": 1
}]
}]
I want the count for each value associated with arr_vs items.
Expected output:
{
"arr_vs":{
"one":[
{
"val":5,
"total_count":2
},{
"val":2,
"total_count":
}
}],
"two":[
{
"val":5,
"total_count":2
},{
"val":2,
"total_count":
}
}]
}
}
Any help will be appreciated.

Answer 1

Outputting to named keys is never really the fantastic thing some people seem to think it is. Realistically I usually want to work with the results returned, and therefore a "list/array" makes a lot more sense.
This is basically every new person basically gets told to abandon their "named keys" concepts, and realize they are working with a database and the inherent problems with named keys. Kind of also why collections are essentially "lists" as well.
So you would be better off getting used to the concept:
db.collection.aggregate([
{ "$unwind": "$arr_vs" },
{ "$group": {
"_id": { "id": "$arr_vs.arr_id", "val": "$arr_vs.val" },
"total_count": { "$sum": 1 }
}},
{ "$group": {
"_id": "$_id.id",
"v": {
"$push": {
"val": "$_id.val",
"total_count": "$total_count"
}
}
}}
])
Which is basically going to give you:
/* 1 */
{
"_id" : "two",
"v" : [
{
"val" : 2.0,
"total_count" : 1.0
},
{
"val" : 5.0,
"total_count" : 1.0
}
]
}
/* 2 */
{
"_id" : "one",
"v" : [
{
"val" : 5.0,
"total_count" : 2.0
}
]
}
/* 3 */
{
"_id" : "three",
"v" : [
{
"val" : 1.0,
"total_count" : 1.0
}
]
}
And is the aggregated data in an iterable and easy to use form.
If you are intent on your output format and have at least a MongoDB 3.4.4 version, you can take that further by compacting the documents and using $arrayToObject:
db.collection.aggregate([
{ "$unwind": "$arr_vs" },
{ "$group": {
"_id": { "id": "$arr_vs.arr_id", "val": "$arr_vs.val" },
"total_count": { "$sum": 1 }
}},
{ "$group": {
"_id": "$_id.id",
"v": {
"$push": {
"val": "$_id.val",
"total_count": "$total_count"
}
}
}},
{ "$group": {
"_id": null,
"arr_vs": {
"$push": {
"k": "$_id",
"v": "$v"
}
}
}},
{ "$project": {
"_id": 0,
"arr_vs": { "$arrayToObject": "$arr_vs" }
}}
])
Or even just apply the final "reshape" client side, if your MongoDB version does not support the new operator:
db.collection.aggregate([
{ "$unwind": "$arr_vs" },
{ "$group": {
"_id": { "id": "$arr_vs.arr_id", "val": "$arr_vs.val" },
"total_count": { "$sum": 1 }
}},
{ "$group": {
"_id": "$_id.id",
"v": {
"$push": {
"val": "$_id.val",
"total_count": "$total_count"
}
}
}},
{ "$group": {
"_id": null,
"arr_vs": {
"$push": {
"k": "$_id",
"v": "$v"
}
}
}},
/*
{ "$project": {
"_id": 0,
"arr_vs": { "$arrayToObject": "$arr_vs" }
}}
*/
]).map( d => ({
"arr_vs": d.arr_vs.reduce((acc,curr) =>
Object.assign(acc,({ [curr.k]: curr.v })),{})
}))
And both produce the same output:
{
"arr_vs" : {
"two" : [
{
"val" : 2.0,
"total_count" : 1.0
},
{
"val" : 5.0,
"total_count" : 1.0
}
],
"one" : [
{
"val" : 5.0,
"total_count" : 2.0
}
],
"three" : [
{
"val" : 1.0,
"total_count" : 1.0
}
]
}
}