Aggregate (group by) query in MongoDB by 2 fields - mongodb

Question

I am using MongoDB. My collection object structure is like the following:
{
"_id" : ObjectId("5a58800acebcda57188bf0aa"),
"title" : "Article title",
"categories" : "politics",
"url" : "https://example.com",
"article_date" : ISODate("2018-01-11T10:00:00.000Z"),
"content" : "content here..."
},
{
"_id" : ObjectId("5a58800acebcda57188bf0aa"),
"title" : "Article title 2",
"categories" : "economics",
"url" : "https://example.com",
"article_date" : ISODate("2018-01-12T10:00:00.000Z"),
"content" : "content here..."
}
Articles are publishing each day and I have many categories.
How can I group the data by date and count documents by specific category, for example:
{
"date": ISODate("2018-01-11T10:00:00.000Z"),
"result": [{
"category": "politics",
"count": 2
}, {
"category": "economics",
"count": 1
}]
},
{
"date": ISODate("2018-01-12T10:00:00.000Z"),
"result": [{
"category": "politics",
"count": 2
}, {
"category": "economics",
"count": 1
}]
}
Thank you in advance

Answer 1

you need to $group twice to get the result, first by article_date and categories then $group on article_date
db.art.aggregate([
{$group : {
_id : {article_date : "$article_date", categories : "$categories"},
count : {$sum : 1}
}},
{$group : {
_id : {article_date : "$_id.article_date"},
result : {$push : {category : "$_id.categories", count : "$count"}}
}},
{$addFields :{
_id : "$_id.article_date"
}}
]).pretty()
result for sample data in question
{
"_id" : ISODate("2018-01-11T10:00:00Z"),
"result" : [
{
"category" : "politics",
"count" : 1
}
]
}
{
"_id" : ISODate("2018-01-12T10:00:00Z"),
"result" : [
{
"category" : "economics",
"count" : 1
}
]
}