如何到达mongodb中的多维数组数据？

Question

我想从以下输出中获取所有 'username'：

> doc=db.instagram1.findOne();
{
        "_id" : ObjectId("55bfba2bc24e980ec46f59c2"),
        "meta" : {
                "code" : NumberLong(200)
        },
        "data" : [
                {
                        "username" : "lifteds0ul",
                        "profile_picture" : "https://igcdn-photos-b-a.akamaihd.net/hphotos-ak-xfa1/t51.2885-19/s150x150/11249117_1590613231201553_906635879_a.jpg",
                        "id" : "2139603325",
                        "full_name" : "River Sage"
                },
                {
                        "username" : "noony__alhosawy",
                        "profile_picture" : "https://igcdn-photos-a-a.akamaihd.net/hphotos-ak-xaf1/t51.2885-19/s150x150/11326999_1464806933841520_1635942624_a.jpg",
                        "id" : "2092127241",
                        "full_name" : "NEAMA"
                },
                {
                        "username" : "_perverse",
                        "profile_picture" : "https://igcdn-photos-g-a.akamaihd.net/hphotos-ak-xaf1/t51.2885-19/s150x150/11312547_878996775481318_911042248_a.jpg",
                        "id" : "2090297101",
                        "full_name" : ""
                },
etc....

在数据值范围内

我的尝试：

> for (data in doc) print(username);
data
data
data
> for (username in doc) print(username);
_id
meta
data
> for (data[username] in doc) print(username);
data
data
data
> for (data:[username] in doc) print(username);
2015-09-06T06:57:34.148+0000 SyntaxError: Unexpected token :

预期输出：

lifteds0ul
noony__alhosawy
_perverse
...

作为第二个警告，如果我能得到 'username' 的计数作为第二个单独的查询，那将非常有帮助。

Answer 1

您只需要 "aggregation" and two projections. In the first $project stage you use the $map operator which returns the array of "username" and; in the second you need to use the $size 运算符即可获得数组中 "username" 的计数。

db.instagram1.aggregate([{ 
    "$project": { 
        "usernames": { 
            "$map": { 
                "input": "$data",
                "as": "d", 
                "in": "$$d.username"
            }
        }
    }},
    { 
        "$project": {
            "usernames": 1, 
            "_id": 0, 
            "count": { "$size": "$usernames" }
    }}
])

哪个returns:

{
        "usernames" : [
                "lifteds0ul",
                "noony__alhosawy",
                "_perverse"
        ],
        "count" : 3
}

如何到达mongodb中的多维数组数据？

how to reach multidimensional array data in mongodb?

mongodb

aggregation-framework