嵌套数组的MongoDB投影

Question

我有一个集合"帐户",其中包含类似于此结构的文档:

{
    "email" : "john.doe@acme.com",
    "groups" : [
        {
            "name" : "group1",
            "contacts" : [
                { "localId" : "c1", "address" : "some address 1" },
                { "localId" : "c2", "address" : "some address 2" },
                { "localId" : "c3", "address" : "some address 3" }
            ]
        },
        {
            "name" : "group2",
            "contacts" : [
                { "localId" : "c1", "address" : "some address 1" },
                { "localId" : "c3", "address" : "some address 3" }
            ]
        }
    ]
}

通过

q = { "email" : "john.doe@acme.com", "groups" : { $elemMatch: { "name" : "group1" } } }
p = { "groups.name" : 0, "groups" : { $elemMatch: { "name" : "group1" } } }
db.accounts.find( q, p ).pretty()

我将成功获得我感兴趣的指定帐户的组.

问题:如何在指定"帐户"的某个"组"中获得有限的"联系人"列表？我们假设我有以下参数:

• 帐号:电子邮件 - "john.doe@acme.com"
• group:name - "group1"
• contact:localIds数组 - ["c1","c3","Not existing id"]

鉴于这些论点,我希望得到以下结果:

{
    "groups" : [
        {
            "name" : "group1", (might be omitted)
            "contacts" : [
                { "localId" : "c1", "address" : "some address 1" },
                { "localId" : "c3", "address" : "some address 3" }
            ]
        }
    ]
}

除了最终的联系人之外,我不需要任何其他内容.

途径

为简单起见,所有查询都尝试仅提取一个匹配的联系人而不是匹配的联系人列表.我尝试了以下查询但没有成功:

p = { "groups.name" : 0, "groups" : { $elemMatch: { "name" : "group1", "contacts" : { $elemMatch: { "localId" : "c1" } } } } }
p = { "groups.name" : 0, "groups" : { $elemMatch: { "name" : "group1", "contacts.localId" : "c1" } } }
not working: returns whole array or nothing depending on localId


p = { "groups.$" : { $elemMatch: { "localId" : "c1" } } }
error: {
    "$err" : "Can't canonicalize query: BadValue Cannot use $elemMatch projection on a nested field.",
    "code" : 17287
}


p = { "groups.contacts" : { $elemMatch: { "localId" : "c1" } } }
error: {
    "$err" : "Can't canonicalize query: BadValue Cannot use $elemMatch projection on a nested field.",
    "code" : 17287
}

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Answer 1

2017年更新

如此好的问题值得现代回应.所请求的数组过滤实际上可以通过简单$match和$project流水线阶段在3.2版的现代MongoDB版本中完成,就像原始的普通查询操作所要求的那样.

db.accounts.aggregate([
  { "$match": {
    "email" : "john.doe@acme.com",
    "groups": {
      "$elemMatch": { 
        "name": "group1",
        "contacts.localId": { "$in": [ "c1","c3", null ] }
      }
    }
  }},
  { "$addFields": {
    "groups": {
      "$filter": {
        "input": {
          "$map": {
            "input": "$groups",
            "as": "g",
            "in": {
              "name": "$$g.name",
              "contacts": {
                "$filter": {
                  "input": "$$g.contacts",
                  "as": "c",
                  "cond": {
                    "$or": [
                      { "$eq": [ "$$c.localId", "c1" ] },
                      { "$eq": [ "$$c.localId", "c3" ] }
                    ]
                  } 
                }
              }
            }
          }
        },
        "as": "g",
        "cond": {
          "$and": [
            { "$eq": [ "$$g.name", "group1" ] },
            { "$gt": [ { "$size": "$$g.contacts" }, 0 ] }
          ]
        }
      }
    }
  }}
])

这使得$filter和$map运算符只能在满足条件的情况下从数组中返回元素,并且性能远远优于使用$unwind.由于管道阶段有效地反映了操作中"查询"和"项目"的结构.find(),因此这里的性能与此类操作基本相同.

请注意,如果目的是实际"跨文档"工作以将详细信息从"多个"文档而不是"一个"中提取出来,那么这通常需要某种类型的$unwind操作才能这样做,因此启用数组项可以进行"分组".

这基本上是方法:

db.accounts.aggregate([
    // Match the documents by query
    { "$match": {
        "email" : "john.doe@acme.com",
        "groups.name": "group1",
        "groups.contacts.localId": { "$in": [ "c1","c3", null ] },
    }},

    // De-normalize nested array
    { "$unwind": "$groups" },
    { "$unwind": "$groups.contacts" },

    // Filter the actual array elements as desired
    { "$match": {
        "groups.name": "group1",
        "groups.contacts.localId": { "$in": [ "c1","c3", null ] },
    }},

    // Group the intermediate result.
    { "$group": {
        "_id": { "email": "$email", "name": "$groups.name" },
        "contacts": { "$push": "$groups.contacts" }
    }},

    // Group the final result
    { "$group": {
        "_id": "$_id.email",
        "groups": { "$push": {
            "name": "$_id.name",
            "contacts": "$contacts" 
        }}
    }}
])

这是对多个匹配的"数组过滤",其基本投影功能.find()无法做到.

您有"嵌套"数组,因此需要处理$unwind两次.与其他操作一起.

Answer 2

您可以使用聚合框架的$unwind运算符。例如：

db.contact.aggregate({$unwind:'$groups'}, {$unwind:'$groups.contacts'}, {$match:{email:'john.doe@acme.com', 'groups.name':'group1', 'groups.contacts.localId':{$in:['c1', 'c3', 'whatever']}}});

应该给出以下结果：

{ "_id" : ObjectId("5500103e706342bc096e2e14"), "email" : "john.doe@acme.com", "groups" : { "name" : "group1", "contacts" : { "localId" : "c1", "address" : "some address 1" } } }
{ "_id" : ObjectId("5500103e706342bc096e2e14"), "email" : "john.doe@acme.com", "groups" : { "name" : "group1", "contacts" : { "localId" : "c3", "address" : "some address 3" } } }

如果您只需要一个对象，则可以使用$group运算符。