How to regroup (or merge) objects in aggregation pipeline in MongoDB? - mongodb

Question

I currently have an aggregation pipeline:
db.getCollection('forms').aggregate([
{ $unwind: //unwind },
{
$match: {
//some matches
}
},
{
$project: {
//some projections
}
},
{
//Finally, im grouping the results
$group: {
_id: {
year: { $year: '$createdAt' },
month: { $month: '$createdAt' },
raceEthnicity: '$demographic.raceEthnicity'
},
count: { $sum: 1 },
}
]
My current results are similar to:
[{
"_id" : {
"year" : 2020,
"month" : 11,
"raceEthnicity" : "Asian"
},
"count" : 1.0
},
{
"_id" : {
"year" : 2020,
"month" : 11,
"raceEthnicity" : "Multiracial"
},
"count" : 3.0
},
{
"_id" : {
"year" : 2020,
"month" : 11,
"raceEthnicity" : "White"
},
"count" : 3.0
},
{
"_id" : {
"year" : 2020,
"month" : 10,
"raceEthnicity" : "White"
},
"count" : 33.0
}]
Is there a way to add a new stage on the pipeline to "merge" results of the same year/month into a single object?
I want to achieve something like:
{
"_id" : {
"year" : 2020,
"month" : 11,
},
"Asian" : 1.0,
"Multiracial": 3.0,
"White": 1.0
},
{
"_id" : {
"year" : 2020,
"month" : 10,
},
"White": 33
}
Is it possible? How can I do that?

Answer 1

Add this one to your aggregation pipeline.
db.collection.aggregate([
{ $set: { "data": { k: "$_id.raceEthnicity", v: "$count" } } },
{ $group: { _id: { year: "$_id.year", month: "$_id.month" }, data: { $push: "$data" } } },
{ $set: { "data": { $arrayToObject: "$data" } } },
{ $replaceRoot: { newRoot: { $mergeObjects: ["$$ROOT", "$data"] } } },
{ $unset: "data" }
])
Unlike the solution from #wak786 you don't need to know all ethnicity at design time. It works for arbitrary ethnicity.

Answer 2

Add these stages to your pipeline.
db.collection.aggregate([
{
$replaceRoot: {
newRoot: {
"$mergeObjects": [
"$$ROOT",
{
$arrayToObject: [
[
{
k: "$_id.raceEthnicity",
v: "$count"
}
]
]
}
]
}
}
},
{
"$group": {
"_id": {
year: "$_id.year",
month: "$_id.month",
},
"Asian": {
"$sum": "$Asian"
},
"Multiracial": {
"$sum": "$Multiracial"
},
"White": {
"$sum": "$White"
}
}
}
])
Below is the mongo playground link. I have taken the current result of your pipeline as input to my query.
Try it here