Mongodb 查找与聚合匹配返回一个空数组

Posted 2023-03-11

【中文标题】Mongodb 查找与聚合匹配返回一个空数组

【英文标题】：Mongodb lookup with match in aggregate returns an empty array

【发布时间】：2020-06-24 13:55:38

【问题描述】：

这是我的用户文档

   "_id":"02a33b9a-284c-4869-885e-d46981fdd679",
   "context":
      "email":"[e-mail]",
      "firstName":"John",
      "lastName":"Smith",
      "company":[
         "e2467c93-114b-4613-a842-f311a8c537b3"
      ],
   ,

和一份公司文件

   "_id":"e2467c93-114b-4613-a842-f311a8c537b3",
   "context":
      "name":"Coca Cola",
      "image":"someimage",
   ,
;

这是我对用户的查询

let users = await Persons.aggregate(
            [$project:
            
                name: $concat: ['$context.firstName', ' ', '$context.lastName'],
                companyId: $arrayElemAt: ['$context.company', 0]
            ,
            $match: name: searchRegExp,
            $lookup: from: 'companies', let: company_id: $arrayElemAt: ['$context.company', 0], pipeline:
            [
                
                    $match: 
                        $expr: 
                            $eq: ['$_id', '$$company_id']
                        
                    
                ,
                
                    $project: name: '$context.name'
                
            ],
            as: 'company'
            ]).toArray()

当我运行这个查询时，我将公司字段作为一个空数组，我在这里做错了什么？

【参考方案1】：

您的第一个管道阶段$project 仅输出_id、name 和companyId，因此当您尝试在$context.company 中引用$context.company 时将有一个空值。你可以改用$addFields：

    $addFields: 
        name: 
            $concat: [
                "$context.firstName",
                " ",
                "$context.lastName"
            ]
        ,
        companyId: 
            $arrayElemAt: [
                "$context.company",
                0
            ]
        
    

Mongo Playground

【参考方案2】：

当您添加字段companyId: $arrayElemAt: ['$context.company', 0] 时，您可以使用$lookup 的简单版本。无需设置两次，一次为companyId: ...，一次为let: company_id: ...

db.user.aggregate([
   
      $addFields: 
         name:  $concat: ["$context.firstName", " ", "$context.lastName"] ,
         companyId:  $arrayElemAt: ["$context.company", 0]  
      
   ,
   
      $lookup: 
         from: "company",
         localField: "companyId",
         foreignField: "_id",
         as: "company"
      
   
])