mongodb count number of documents for every category - mongodb

Question

My collection looks like this:
{
"_id":ObjectId("5744b6cd9c408cea15964d18"),
"uuid":"bbde4bba-062b-4024-9bb0-8b12656afa7e",
"version":1,
"categories":["sport"]
},
{
"_id":ObjectId("5745d2bab047379469e10e27"),
"uuid":"bbde4bba-062b-4024-9bb0-8b12656afa7e",
"version":2,
"categories":["sport", "shopping"]
},
{
"_id":ObjectId("5744b6359c408cea15964d15"),
"uuid":"561c3705-ba6d-432b-98fb-254483fcbefa",
"version":1,
"categories":["politics"]
}
I want to count the number of documents for every category. To do this, I unwind the categories array:
db.collection.aggregate(
{$unwind: '$categories'},
{$group: {_id: '$categories', count: {$sum: 1}} }
)
Result:
{ "_id" : "sport", "count" : 2 }
{ "_id" : "shopping", "count" : 1 }
{ "_id" : "politics", "count" : 1 }
Now I want to count the number of documents for every category, but where document version is the latest version.
This is where I am stuck.

Answer 1

It's ugly but I think this gives you what you're after:
db.collection.aggregate(
{ $unwind : "$categories" },
{ $group :
{ "_id" : { "uuid" : "$uuid" },
"doc" : { $push : { "version" : "$version", "category" : "$categories" } },
"maxVersion" : { $max : "$version" }
}
},
{ $unwind : "$doc" },
{ $project : { "_id" : 0, "uuid" : "$id.uuid", "category" : "$doc.category", "isCurrentVersion" : { $eq : [ "$doc.version", "$maxVersion" ] } } },
{ $match : { "isCurrentVersion" : true }},
{ $group : { "_id" : "$category", "count" : { $sum : 1 } } }
)

Answer 2

You can do this by first grouping the denormalized documents (from the $unwind operator step) by two keys, i.e. the categories and version fields. This is necessary for the preceding pipeline step which orders the grouped documents and their accumulated counts by the version (desc) and categories (asc) keys respectively using the $sort operator.
Another grouping will be required to get the top documents in each categories group after ordering using the $first operator. The following shows this
db.collection.aggregate(
{ "$unwind": "$categories" },
{
"$group": {
"_id": {
'categories': '$categories',
'version': '$version'
},
"count": { "$sum": 1 }
}
},
{ "$sort": { "_id.version": -1, "_id.categories": 1 } },
{
"$group": {
"_id": "$_id.categories",
"count": { "$first": "$count" },
"version": { "$first": "$_id.version" }
}
}
)
Sample Output
{ "_id" : "shopping", "count" : 1, "version" : 2 }
{ "_id" : "sport", "count" : 1, "version" : 2 }
{ "_id" : "politics", "count" : 1, "version" : 1 }