Count articles grouping by tags mongodb - mongodb

Question

I had a lot of articles with a field called tags, and is an array of tags _ids, and for statistics purpose I want to count how many articles we had by each tag. If tags were a simple tag _id, it's easy because I could group by tag, but is an array of tags, and I can't group by that field.
First I try with this:
db.note.aggregate([{$match: {
publishedAt: {
$gte: ISODate('2018-01-01'),
$lte: ISODate('2019-01-01')
}
}}, {$group: {
_id: "$tags",
"total": {
"$sum": 1
}
}}, {$lookup: {
from: 'tags',
localField: '_id',
foreignField: '_id',
as: 'tag'
}}, {$unwind: {
path: "$tag"
}}, {$project: {
total: 1,
"tag.name": 1
}}, {$sort: {
total: -1
}}])
But that doesn't work, that query, group by tags group, so I try to do this:
{
'$match': {
'publishedAt': {
'$gte': new Date(req.body.gte),
'$lte': new Date(req.body.lte)
}
}
},
{
'$unwind': {
'path': '$tags'
}
}, {
'$group': {
'_id': '$tags',
'total': {
'$sum': 1
}
}
}, {
'$lookup': {
'from': 'tags',
'localField': '_id',
'foreignField': '_id',
'as': 'tag'
}
}, {
'$project': {
'total': 1,
'tag.name': 1
}
}, {
'$sort': {
'total': -1
}
},
{
'$unwind': {
'path': '$tag'
}
}
)
But the problem with this, that group for the first tag from the array and I miss all other tags in that array.
What do you think will be the solution?

Answer 1

I had a lot of articles with a field called tags, and is an array of
tags _ids, and for statistics purpose I want to count how many
articles we had by each tag.
You can try this (I am assuming the following input documents):
notes:
{ _id: 1, name: "art-1", author: "ab", tags: [ "t1", "t2" ] },
{ _id: 2, name: "art-2", author: "cd", tags: [ "t1", "t3" ] },
{ _id: 3, name: "art-3", author: "wx", tags: [ "t4", "t3" ] },
{ _id: 4, name: "art-4", author: "yx", tags: [ "t1" ] }
tags:
{ _id: 1, id: "t1", name: "t1's name" },
{ _id: 2, id: "t2", name: "t2's name" },
{ _id: 3, id: "t3", name: "t3's name" },
{ _id: 4, id: "t4", name: "t4's name" }
The Query:
db.tags.aggregate( [
{
$lookup: {
from: "notes",
localField: "id",
foreignField: "tags",
as: "tag_matches"
}
},
{ $project: { id: 1, name: 1, _id: 0, count: { $size: "$tag_matches" } } }
] )
The Output:
{ "id" : "t1", "name" : "t1's name", "count" : 3 }
{ "id" : "t2", "name" : "t2's name", "count" : 1 }
{ "id" : "t3", "name" : "t3's name", "count" : 2 }
{ "id" : "t4", "name" : "t4's name", "count" : 1 }