mongodb - 在 mongodb 的 $lookup 中应用条件

Question

我是 mongoDb 的新手。我已经设置了两个集合。 1) 书 2) 评论

book : 
_id
title
author
posted
price

评论是:

_id
bookId
comment
status

我想获取那些具有status = 1 的书的评论。

这个我试过了。

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."});
    });
});

当我调用我的 API 时，我在 postman 中得到了它。

{
"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": []
    }
]
}

我需要评论具有状态 1 值的数据。我尝试在 $lookup 之后使用 $match 但它不起作用。我也尝试过使用 $eq 这对我也不起作用。由于我刚刚开始学习 mongodb，所以可能我以错误的方式设置它。

Answer 1

来自 MongoDB v3.6.3之后，最快的查询性能将像这样实现:

确保您在 comments 集合中的 bookId 和 status 字段上有一个索引:

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

然后使用 $lookup 阶段的新 pipeline 属性 ( documentation ):

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