The data in the collection is as follows:

{
 _id: ObjectId(7df78ad8902c)
 title: 'MongoDB Overview',
 description: 'MongoDB is no sql database',
 by_user: 'runoob.com',
 url: 'http://www.runoob.com',
 tags: ['mongodb', 'database', 'NoSQL'],
 likes: 100
 },
 {
 _id: ObjectId(7df78ad8902d)
 title: 'NoSQL Overview',
 description: 'No sql database is very fast',
 by_user: 'runoob.com',
 url: 'http://www.runoob.com',
 tags: ['java', 'php', 'go'],
 likes: 10
 },
 {
 _id: ObjectId(7df78ad8902e)
 title: 'Neo4j Overview',
 description: 'Neo4j is no sql database',
 by_user: 'Neo4j',
 url: 'http://www.neo4j.com',
 tags: ['neo4j', 'database', 'NoSQL'],
 likes: 750
 },

Now we calculate the number of articles written by each author through the above set, and use aggregate () to calculate the result as follows:

> db.mycol.aggregate([{$group : {_id : "$by_user", num_tutorial : {$sum : 1}}}])
 {
 "result" : [
 {
 "_id" : "runoob.com",
 "num_tutorial" : 2
 },
 {
 "_id" : "Neo4j",
 "num_tutorial" : 1
 }
 ],
 "ok" : 1
 }
 >

The above example is similar to sql statement:

select by_user, count(*) from mycol group by by_user

In the above example, we grouped the data through the field by_user and calculated the sum of the same values in the by_user field

Here is how to get the length of the array

{
 "history_id":  [1, 2, 3],
 "author": "andy",
 ...
 }
 db.test.insert({username:"Alex", tags: ['C#', 'Java', 'C++'] });
 db.test.aggregate(
 {$match: {username : "Alex"}},
 {$unwind: "$tags"},
 {$project: {count:{$add:1}}},
 {$group: {_id: null, number: {$sum: "$count" }}}
 );
 { "result" : [ { "_id" : null, "number" : 3 } ], "ok" : 1 }

The question is, if I want to calculate the tags length according to the original grouping, how do I do it?