Mongodb聚合操作中$lookup的参数必须为字符串。

Question

Mongodb聚合操作中$lookup的参数必须为字符串。

mongodbmongodb-queryaggregation-framework

10

        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"
         }
    }
])

我正在尝试在聚合查询的查找中使用管道。虽然代码与MongoDB文档中的代码相同，但仍然出现错误。

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"
}

我正在使用mongodb v3.4。

- Nakarmi

2个回答

-2

在 $lookup 中，您可以先加入集合并进行匹配、投影等操作，然后再进行后续的操作。同时，您需要确保外键字段引用了 holidays 集合。

例如：

{
  $lookup:
     {
       from: "holidays",
       localField: "holidaysID", //yourLocalfield, it is local field in absences. you should have it to join 2 collections
       foreignField : "_id", //theforeignfield, it is _id in the holidays
       as: "holidays"
     }
},{ $match: { year: 2018 } },
{ $project: { _id: 0, date: { name: "$name", date: "$date" } } },
{ $replaceRoot: { newRoot: "$date" } }

希望能有所帮助。

- dundun

这取决于MongoDB的版本。请查看文档 https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#join-conditions-and-uncorrelated-sub-queries (-; - Neodan

谢谢您的回答，但这并不能解决我的问题。$lookup从holidays中获取所有文档，而我需要从一个文档中仅获取所选数据。 - Nakarmi

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- Neodan · Accepted Answer

由于您正在尝试在MongoDB v3.4上使用$lookup功能（语法），因此需要注意：

MongoDB v3.4 $lookup 语法如下：

{
   $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 语法如下：

{
   $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/aggregation/lookup/