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?
Or other methods, is there any idea, thank you