MongoDB count occurances with group and unwind - mongodb

Question

I have a MongoDB database with the following document structure:
{
"name": "ServiceA",
"areas": ["X", "Y", "Z"],
"tags": [
{
"name": "Financial",
"type": "A"
},
{
"name": "Consumer",
"type": "B"
}
]
}
There's many entries each with the same structure. Containing the same areas.
There's many predefined tag names, sorted into a few types.
The aim is to group by area and then count the number of occurrences of each tag. So an output like this:
{
"area": "X",
"count": 100, // Total entries with X as an area
"tagNameCount": {
"Financial": 20,
"Consumer": 10,
...
},
"tagTypeCount": {
"A": 70,,
"B: 40
}
}
I've been starting of using $unwind on areas, but it's the next steps from there I'm stuck on. I get that I need to use $group, but I can't work out how to count occurrences.

Answer 1

You may use $facet operator which allows perform several aggregation in one.
Walkthrough
1. We $unwind by area and tags
2. With $facet, we perform 3 parallel aggregations:
2.1 We count unique areas
2.2 We count unique tag names for each area
2.3 We count unique tag type for each area
3. We join 2 parallel arrays by flatten areas
4. We assemble desired output
db.collection.aggregate([
{
$unwind: "$areas"
},
{
$unwind: "$tags"
},
{
$facet: {
areas: [
{
$group: {
_id: "$areas",
count: {
$addToSet: "$_id"
}
}
},
{
$project: {
_id: 0,
area: "$_id",
count: {
$size: "$count"
}
}
}
],
tagNameCount: [
{
$group: {
_id: {
name: "$tags.name",
areas: "$areas"
},
count: {
$addToSet: "$_id"
}
}
},
{
$group: {
_id: "$_id.areas",
tagNameCount: {
$push: {
k: "$_id.name",
v: {
$size: "$count"
}
}
}
}
},
{
$addFields: {
tagNameCount: {
$arrayToObject: "$tagNameCount"
}
}
}
],
tagTypeCount: [
{
$group: {
_id: {
type: "$tags.type",
areas: "$areas"
},
count: {
$addToSet: "$_id"
}
}
},
{
$group: {
_id: "$_id.areas",
tagTypeCount: {
$push: {
k: "$_id.type",
v: {
$size: "$count"
}
}
}
}
},
{
$addFields: {
tagTypeCount: {
$arrayToObject: "$tagTypeCount"
}
}
}
]
}
},
{
$unwind: "$areas"
},
{
$addFields: {
"tagNameCount": {
$filter: {
input: "$tagNameCount",
cond: {
$eq: [
"$areas.area",
"$$this._id"
]
}
}
},
"tagTypeCount": {
$filter: {
input: "$tagTypeCount",
cond: {
$eq: [
"$areas.area",
"$$this._id"
]
}
}
}
}
},
{
$project: {
area: "$areas.area",
count: "$areas.count",
tagNameCount: {
$arrayElemAt: [
"$tagNameCount.tagNameCount",
0
]
},
tagTypeCount: {
$arrayElemAt: [
"$tagTypeCount.tagTypeCount",
0
]
}
}
},
{
$sort: {
area: 1
}
}
])
MongoPlayground

Answer 2

Here's one method:
unwind both areas and tags
for each area collect the applicable tags, and the unique names and types
count the names to get the total number of tags
for each unique name, count the matching values in the tags
do the same for each unique type
project out the unique fields
db.collection.aggregate([
{$unwind: "$areas"},
{$unwind: "$tags"},
{$group: {
_id: "$areas",
names: {$push: "$tags.name"},
uniqueNames: {$addToSet: "$tags.name"},
types: {$push: "$tags.type"},
uniqueTypes: {$addToSet: "$tags.type"}
}},
{$addFields: {
count: {$size: "$names"},
names: {
$arrayToObject: {
$map: {
input: "$uniqueNames",
as: "needle",
in: {
k: "$$needle",
v: {
$size: {
$filter: {
input: "$names",
cond: {$eq: ["$$this","$$needle"]}
}}}}}}},
types: {
$arrayToObject: {
$map: {
input: "$uniqueTypes",
as: "needle",
in: {
k: "$$needle",
v: {$size: {
$filter: {
input: "$types",
cond: { $eq: [ "$$this","$$needle"]}
}}}}}}}}},
{
$project: {
uniqueNames: 0,
uniqueTypes: 0
}}
])
Playground