Obtaining $group result with group count

Question

Assuming I have a collection called \"posts\" (in reality it is a more complex collection, posts is too simple) with the following structure:

> db.posts.f         


        

Answer 1

1. Use $project to save tag and count into tmp
2. Use $push or addToSet to store tmp into your data list.

Code:

db.test.aggregate(
    {$unwind: '$tags'}, 
    {$group:{_id: '$tags', count:{$sum:1}}},
    {$project:{tmp:{tag:'$_id', count:'$count'}}}, 
    {$group:{_id:null, total:{$sum:1}, data:{$addToSet:'$tmp'}}}
)

Output:

{
    "result" : [
            {
                    "_id" : null,
                    "total" : 5,
                    "data" : [
                            {
                                    "tag" : "SOME",
                                    "count" : 1
                            },
                            {
                                    "tag" : "RANDOM",
                                    "count" : 2
                            },
                            {
                                    "tag" : "TAGS1",
                                    "count" : 1
                            },
                            {
                                    "tag" : "TAGS",
                                    "count" : 1
                            },
                            {
                                    "tag" : "SOME1",
                                    "count" : 1
                            }
                      ]
              }
      ],
      "ok" : 1
}

Answer 2

I'm not sure you need the aggregation framework for this other than counting all the tags eg:

db.posts.aggregate(
  { "unwind" : "$tags" },
  { "group" : {
      _id: { tag: "$tags" },
      count: { $sum: 1 }
  } }
);

For paginating through per tag you can just use the normal query syntax - like so:

db.posts.find({tags: "RANDOM"}).skip(10).limit(10)