计算符合给定条件的子文档元素 [英] Count Elements SubDocument that match a given criterion

问题描述

我在 mongodb 中有以下文档结构

I have the following document structure in mongodb

{
    "_id" : "123",
    "first_name" : "Lorem",
    "last_name" : "Ipsum",
    "conversations" : {
            "personal" : [
                    {
                            "last_message" : "Hello bar",
                            "last_read" : 1474456404
                    },
                     {
                            "last_message" : "Hello foo",
                            "last_read" : 1474456404
                    },
                    ...
            ],

            "group" : [
                    {
                            "last_message" : "Hello Everyone",
                            "last_read" : null
                    }
                    ...
            ]
    }
}

我想计算给定用户的 last_read 为空的子数组 personal 和 group 的对话数.请问我怎样才能做到这一点?

I want to count the number of conversations from the sub arrays, personal and group where the last_read is null, for a given user. Please how can I achieve this?

我试过了:

db.messages.aggregate(
   [
    { $match: {"_id":"123", 'conversations.$.last_read': null }},
      {
         $group: {
            {$size: "$conversations.personal"}, {$size: "$conversations.group"}
         }
      }
   ]
);

但没有得到他想要的输出.请问有更好的主意吗?

but didn't get he desired output. Any better ideas, please?

推荐答案

下面的查询统计personal和group数组下有的子文档个数last_read 值 null.

The following query counts the number of sub documents under personal and group arrays that have last_read value null.

$concatArrays 将多个数组组合成一个数组.它是在 MongoDB 3.2 中引入的.

$concatArrays combines multiple arrays into a single one. It was introduced in MongoDB 3.2.

db.collection.aggregate([
                        { "$match": {"_id":"123", 'conversations.$.last_read': null }},
                        { "$project":{"messages":{$concatArrays : ["$conversations.personal","$conversations.group"]}}}, 
                        { "$unwind": "$messages"}, {$match:{"messages.last_read": null}}, 
                        { "$group":{"_id":null, count: {$sum:1}}}
                ])

样本结果:

{ "_id" : null, "count" : 3 }

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