$ lookup的Foreign字段可以是嵌套文档的字段吗? [英] The Foreign field of $lookup could be the field of nested document?

问题描述

$ lookup 用于执行左外部联接到同一数据库中未分片的集合中，以从加入的"集合中筛选出文档，以在Mongo中进行处理.

$lookup is used to perform a left outer join to an unsharded collection in the same database to filter in documents from the "joined" collection for processing in Mongo.

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

foreignField可以是from集合的嵌套文档的字段吗?

Could the foreignField be the field of the nested document of from collection?

例如，有以下两个集合.

For example, there are two collections as following.

history集合

 [{
  id:'001',
  history:'today worked',
  child_id:'ch001'
},{
  id:'002',
  history:'working',
  child_id:'ch004'
},{
  id:'003',
  history:'now working'
  child_id:'ch009'
}],

childsgroup集合

[{
  groupid:'g001', name:'group1'
  childs:[{
      id:'ch001',
      info:{name:'a'}
  },{
      id:'ch002',
      info:{name:'a'}
  }]
},{
  groupid:'g002', name:'group1'
  childs:[{
      id:'ch004',
      info:{name:'a'}
  },{
      id:'ch009',
      info:{name:'a'}
  }]
}]

那么，这个aggregation代码可以这样执行吗?

so, this aggregation code could be executed like this?

db.history.aggregate([
   {
     $lookup:
       {
         from: "childsgroup",
         localField: "child_id",
         foreignField: "childs.$.id",
         as: "childinfo"
       }
  }
])

所以我想得到这样的结果.

So I want to get the result like this.

     [{
      id:'001',
      history:'today worked',
      child_id:'ch001',
      childinfo:{
          id:'001',
          history:'today worked',
          child_id:'ch001'
       }
    },  .... ]

这不可能吗?

推荐答案

$ lookup没有位置运算符，但是您可以在MongoDB 3.6中使用自定义pipeline来定义自定义联接

There's no positional operator for $lookup but you can use custom pipeline in MongoDB 3.6 to define custom join conditions:

db.history.aggregate([
    {
        $lookup: {
            from: "childsgroup",
            let: { child_id: "$child_id" },
            pipeline: [
                { $match: { $expr: { $in: [ "$$child_id", "$childs.id" ] } } },
                { $unwind: "$childs" },
                { $match: { $expr: { $eq: [ "$childs.id", "$$child_id" ] } } },
                { $replaceRoot: { newRoot: "$childs" } }
            ],
            as: "childInfo"
        }
    }
])

首先添加了$match以提高性能:我们只想从childsgroup中查找包含匹配的child_id的文档，然后我们可以在$unwind阶段之后匹配子文档.

First $match added to improve performance: we want to find only those documents from childsgroup that contain matching child_id and then we can match subdocuments after $unwind stage.

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