Counting with nested aggregation - mongodb

Question

I have been trying to group and count registration collection data for a stats page, as well as to make for dynamic registration, but I can't get it to count for more than one grouping.
Sample registration collection data:
{
"_id" : ObjectId("58ec60078cc818505fb75ace"),
"event" : "Women's BB",
"day" : "Saturday",
"group" : "nonpro",
"division" : "Women's",
"level" : "BB"
}
{
"_id" : ObjectId("58ec60078cc818505fb75acf"),
"event" : "Coed BB",
"day" : "Sunday",
"group" : "nonpro",
"division" : "Coed",
"level" : "BB"
}
{
"_id" : ObjectId("58ec60098cc818505fb75ad0"),
"event" : "Men's BB",
"day" : "Saturday",
"group" : "nonpro",
"division" : "Men's",
"level" : "BB"
}
{
"_id" : ObjectId("58ec60168cc818505fb75ad1"),
"event" : "Men's B",
"day" : "Saturday",
"group" : "nonpro",
"division" : "Men's",
"level" : "B"
}
{
"_id" : ObjectId("58ec60178cc818505fb75ad2"),
"event" : "Women's Open",
"day" : "Saturday",
"group" : "pro",
"division" : "Women's",
"level" : "Pro"
}
{
"_id" : ObjectId("58ec60188cc818505fb75ad3"),
"event" : "Men's Open",
"day" : "Saturday",
"group" : "pro",
"division" : "Men's",
"level" : "Pro"
}
I'd like to reorganize it and do counts returning something like this:
[ {_id: { day: "Saturday", group: "nonpro" },
count: 3,
divisions: [
{ division: "Men's",
count: 2,
levels: [
{ level: "BB", count: 1 },
{ level: "B", count: 1 }]
},
{ division: "Women's",
count: 1,
levels: [
{ level: "BB", count: 1 }]
}
},
{_id: { day: "Saturday", group: "pro" },
count: 2,
divisions: [
{ division: "Men's",
count: 1,
levels: [
{ level: "Pro", count: 1 }
},
{ division: "Women's",
count: 1,
levels: [
{ level: "Pro", count: 1 }]
}
},
{_id: { day: "Sunday", group: "nonpro" },
count: 1,
divisions: [
{ division: "Coed",
count: 1,
levels: [
{ level: "BB", count: 1 }
}
}]
I know I should be using the aggregate() function, but am having a hard time making it work with the count. Here is what my aggregate looks like so far:
Registration
.aggregate(
{ $group: {
_id: { day: "$day", group: "$group" },
events: { $addToSet: { division: "$division", level: "$level"} },
total: { $sum: 1}
}
})
This returns the total registrations per day/group combination, but if I try adding total: {$sum: 1} to the events set, I just get 1 (which makes sense). Is there a way to make this work in one database call, or do I need to do it separately for each level of grouping I need counts for?

Answer 1

You essentially need 3 levels of $group pipeline stages. The first one will group the documents by all four keys i.e. day, group, division and level. Aggregate the counts for the group
which will be the counts for the level.
The preceding group will take three keys i.e. day, group and division and the aggregate count will sum the previous group counts as well as creating the levels array.
The last group will be the day and group keys + the divisions list embedded with the results from the previous group.
Consider running the following pipeline for the expected results:
Registration.aggregate([
{
"$group": {
"_id": {
"day": "$day",
"group": "$group",
"division": "$division",
"level": "$level"
},
"count": { "$sum": 1 }
}
},
{
"$group": {
"_id": {
"day": "$_id.day",
"group": "$_id.group",
"division": "$_id.division"
},
"count": { "$sum": "$count" },
"levels": {
"$push": {
"level": "$_id.level",
"count": "$count"
}
}
}
},
{
"$group": {
"_id": {
"day": "$_id.day",
"group": "$_id.group"
},
"count": { "$sum": "$count" },
"divisions": {
"$push": {
"division": "$_id.division",
"count": "$count",
"levels": "$levels"
}
}
}
}
], (err, results) => {
if (err) throw err;
console.log(JSON.stringify(results, null, 4));
})
Sample Output
/* 1 */
{
"_id" : {
"day" : "Saturday",
"group" : "nonpro"
},
"count" : 3,
"divisions" : [
{
"division" : "Women's",
"count" : 1,
"levels" : [
{
"level" : "BB",
"count" : 1
}
]
},
{
"division" : "Men's",
"count" : 2,
"levels" : [
{
"level" : "BB",
"count" : 1
},
{
"level" : "B",
"count" : 1
}
]
}
]
}
/* 2 */
{
"_id" : {
"day" : "Saturday",
"group" : "pro"
},
"count" : 2,
"divisions" : [
{
"division" : "Women's",
"count" : 1,
"levels" : [
{
"level" : "Pro",
"count" : 1
}
]
},
{
"division" : "Men's",
"count" : 1,
"levels" : [
{
"level" : "Pro",
"count" : 1
}
]
}
]
}
/* 3 */
{
"_id" : {
"day" : "Sunday",
"group" : "nonpro"
},
"count" : 1,
"divisions" : [
{
"division" : "Coed",
"count" : 1,
"levels" : [
{
"level" : "BB",
"count" : 1
}
]
}
]
}