mongodb中$lookup中的应用条件

Question

我是mongoDb的新手。我已经设置了两个收藏品。( 1)第2卷)评论

book : 
_id
title
author
posted
price

评论意见是：

_id
bookId
comment
status

我想得到那些有status = 1的书的评论。

我试过这个。

return new promise((resolve, reject) => {
    db.collection('book').aggregate([
        {
            $lookup:{
                from:'comments',
                localField: "_id",
                foreignField: "bookId",
                as: "comments"                                                                  
            }
        }      
    ]).toArray().then((result) => {
        if(result.length > 0){
            res.send({ status: 1, message: "Success.", data:result });
        }else{
            res.send({ status: 0, message: "No data found." });
        }
    }).catch((err) => {
        res.send({ status: 0, message: "Something went wrong."});
    });
});

当我打电话给我的API时，我从邮递员那里得到了这个。

{
"status": 1,
"message": "Success.",
"data": [
    {
        "_id": "5bacad201bff841afb40791f",
        "title": "Game of thrones",
        "author": "John snow",
        "posted": "16/07/1995",
        "price": 1000,
        "comments": [
            {
                "_id": "5bacc31aa2d365256cab31ce",
                "bookId": "5bacad201bff841afb40791f",
                "comment": "Winter is comming"
            },
            {
                "_id": "5bacc3f65c716925df953615",
                "bookId": "5bacad201bff841afb40791f",
                "comment": "It has a level of politics"
            },
            {
                "_id": "5bacd60ea38cc526f1fee1d1",
                "bookId": "5bacad201bff841afb40791f",
                "comment": "It has a level of politics",
                "status": 1
            }
        ]
    },
    {
        "_id": "5bacad601bff841afb407920",
        "title": "Breaking bed",
        "author": "Haison burg",
        "posted": "20/08/2002",
        "price": 550,
        "comments": []
    }
]
}

我需要有状态1值的评论的数据。我尝试过在$match之后使用$lookup，但它不起作用。我也尝试过使用$eq，这也不适合我。可能是因为我刚刚开始学习mongodb，所以我把它设置得很错误。

Answer 1

您可以在这里使用$addFields和$filter聚合

db.collection("book").aggregate([
  { "$lookup": {
    "from": "comments",
    "localField": "_id",
    "foreignField": "bookId",
    "as": "comments"                                                                  
  }},
  { "$addFields": {
    "comments": {
      "$filter": {
        "input": "$comments",
        "cond": { "$eq": ["$$this.status", 1] }
      }
    }
  }}
])

Answer 2

从MongoDB v3.6.3开始，最快的查询性能将实现如下所示：

确保在bookId集合中的status字段和comments集合中有一个索引：

db.comments.createIndex({ "bookId": 1, "status": 1 })

然后使用$lookup stage (文档)的新的文档属性：

db.books.aggregate([{
    "$lookup": {
        "from": "comments",
        "let": { "bId": "$_id" },
        "pipeline": [{
            "$match": {
                $expr: { $eq: [ "$bookId", "$$bId" ] },
                "status": 1
            }
        }],
        "as": "comments"
    }
}])