$lookup 的 Mongodb 聚合参数必须是字符串 [英] Mongodb aggregate arguments to $lookup must be strings

问题描述

        db.absences.insert([
           { "_id" : 1, "student" : "Ann Aardvark", sickdays: [ new Date ("2018-05-01"),new Date ("2018-08-23") ] },
           { "_id" : 2, "student" : "Zoe Zebra", sickdays: [ new Date ("2018-02-01"),new Date ("2018-05-23") ] },
        ])

    db.holidays.insert([
       { "_id" : 1, year: 2018, name: "New Years", date: new Date("2018-01-01") },
       { "_id" : 2, year: 2018, name: "Pi Day", date: new Date("2018-03-14") },
       { "_id" : 3, year: 2018, name: "Ice Cream Day", date: new Date("2018-07-15") },
       { "_id" : 4, year: 2017, name: "New Years", date: new Date("2017-01-01") },
       { "_id" : 5, year: 2017, name: "Ice Cream Day", date: new Date("2017-07-16") }
    ])

db.absences.aggregate([
   {
      $lookup:
         {
           from: "holidays",
           pipeline: [
              { $match: { year: 2018 } },
              { $project: { _id: 0, date: { name: "$name", date: "$date" } } },
              { $replaceRoot: { newRoot: "$date" } }
           ],
           as: "holidays"
         }
    }
])

我正在尝试使用管道查找聚合查询.与Mongodb文档中的一样，它仍然给出错误

I'm trying to use pipeline in lookup for aggregation query. Having this as same as from the Mongodb documentation, it still gives an error

Unable to execute the selected commands

Mongo Server error (MongoCommandException): Command failed with error 4570: 'arguments to $lookup must be strings, pipeline: [ { $match: { year: 2018.0 } }, { $project: { _id: 0.0, date: { name: "$name", date: "$date" } } }, { $replaceRoot: { newRoot: "$date" } } ] is type array' on server localhost:27017. 

The full response is:
{ 
    "ok" : 0.0, 
    "errmsg" : "arguments to $lookup must be strings, pipeline: [ { $match: { year: 2018.0 } }, { $project: { _id: 0.0, date: { name: "$name", date: "$date" } } }, { $replaceRoot: { newRoot: "$date" } } ] is type array", 
    "code" : NumberInt(4570), 
    "codeName" : "Location4570"
}

我使用的是 mongodb v3.4.

I'm using mongodb v3.4.

推荐答案

因为您正在尝试使用 $lookup 特性(语法)来自 MongoDB v3.6 在 MongoDB v3.4

Because you are trying to use the $lookup features (syntax) from MongoDB v3.6 on MongoDB v3.4

MongoDB v3.4 $lookup 语法:

{
   $lookup:
     {
       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>
     }
}

MongoDB v3.6 $lookup 语法:

{
   $lookup:
     {
       from: <collection to join>,
       let: { <var_1>: <expression>, …, <var_n>: <expression> },
       pipeline: [ <pipeline to execute on the collection to join> ],
       as: <output array field>
     }
}

https://docs.mongodb.com/manual/reference/operator/聚合/查找/

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