MongoDB match filters with grouping and get total count - mongodb

Question

My sample data:
{
"_id": "random_id_1",
"priority": "P1",
"owners": ["user-1", "user-2"],
},
{
"_id": "random_id_2",
"priority": "P1",
"owners": ["user-1", "user-2"],
},
{
"_id": "random_id_3",
"priority": "P2",
"owners": ["user-1", "user-2"],
},
I want to run an aggregation pipeline on the data involving match filters and grouping, also I want to limit the number of groups returned as well as the number of items in each group.
Essentially, if limit=2, limit_per_group=1, group_by=owner, priority=P1, I want the following results:
[
{
"data": [
{
"group_key": "user-1",
"total_items_in_group": 2,
"limited_items": [
{
"_id": "random_id_1",
"priority": "P1",
"owners": ["user-1", "user-2"],
},
],
},
{
"group_key": "user-2",
"total_items_in_group": 2,
"limited_items": [
{
"_id": "random_id_1",
"priority": "P1",
"owners": ["user-1", "user-2"],
},
],
},
]
},
{
"metadata": {
"total_items_matched": 2,
"total_groups": 2
}
},
]
Need some help on how to write an aggregation pipeline to get the required result.
My current query is as follows:
{
"$match": {
"priority": "P1"
}
},
{
"$facet": {
"data": [
{
$addFields: {
"group_by_owners": "$owners"
}
},
{
$unwind: "$group_by_owners"
},
{
$group: {
"_id": "$group_by_owners",
"total_items_in_group": {
$sum: 1
},
"items": {
$push: "$$ROOT"
}
}
},
{
$sort: {
"total": -1
}
},
{
$unset: "items.group_by_owners"
},
{
$project: {
"_id": 1,
"total_items_in_group": 1,
"limited_items": {
$slice: [
"$items",
1
]
}
}
},
{
"$limit": 2
}
],
"metadata": [
{
$count: "total_items_matched"
}
]
}
}
Mongo playground link
I am unable to calculate the total number of groups.

Answer 1

add new stage of $addfields at the end of pipeline
db.collection.aggregate([
{
"$match": {
"priority": "P1"
}
},
{
"$facet": {
"data": [
{
$addFields: {
"group_by_owners": "$owners"
}
},
{
$unwind: "$group_by_owners"
},
{
$group: {
"_id": "$group_by_owners",
"total_items_in_group": {
$sum: 1
},
"items": {
$push: "$$ROOT"
}
}
},
{
$sort: {
"total": -1
}
},
{
$unset: "items.group_by_owners"
},
{
$project: {
"_id": 0,
"group_key": "$_id",
"total_items_in_group": 1,
"limited_items": {
$slice: [
"$items",
1
]
}
}
},
{
"$limit": 2
}
],
"metadata": [
{
$count: "total_items_matched",
}
]
}
},
{
"$addFields": {
"metadata.total_groups": {
"$size": "$data"
}
}
}
])
https://mongoplayground.net/p/y5a0jvr6fxI