MongoDB：检查数组中是否存在对象，如果存在则返回true，否则返回false。

Question

我想查询集合中的数组，并在数组中存在对象时投影值“true”。如果数组中不存在该对象，则投影“false”。我正在使用MongoDB进行操作，对它不太熟悉。

在我的场景中，我有两个要处理的集合。我正在对“员工”成员集合进行聚合，并使用$lookup函数查询“企业”集合。在“企业”集合中，我有一个包含企业能力的数组。

例如，我有员工集合：

staff = [
  ...
  {_id: 1, businessId: 11},
  {_id: 2, businessId: 22},
  ....
]

企业收集

businesses = [
  ...
  {_id: 11, capabilities: ["claiming", "pushing"]},
  {_id: 22, capabilities: ["claiming", "popping"]},
  ....
]

并且设置了一个类似于$lookup的东西

db.getCollection('staff').aggregate([
    {
        $lookup:
            {
                from: "businesses",
                localField: "businessId",
                foreignField: "_id",
                as: "business_Info"
            }
    },

如果“capabilities”中出现“claiming”，我该如何将每个员工的$project转换为类似$canClaim：true的值？

- Maandeep

Answer 1

您可以使用聚合操作符$in来检查数组是否包含该值。

db.getCollection("staff").aggregate([
  { "$lookup": {
    "from": "businesses",
    "localField": "businessId",
    "foreignField": "_id",
    "as": "business_Info"
  }},
  { "$unwind": "$business_Info" },
  { "$addFields": {
    "canClaim": { "$in": ["claiming", "$business_Info.capabilities"] }
  }}
])

MongoPlayground

- Ashh

---

太棒了！谢谢，我很快就会尝试。出于好奇，$unwind函数是否会影响对$business_Info中调用的字段？ - Maandeep

不，它只会解构你的数组字段。 - Ashh

Answer 2

db.getCollection("staff").aggregate(

    // Pipeline
    [
        // Stage 1
        {
            $lookup: // Equality Match
            {
                from: "businesses",
                localField: "businessId",
                foreignField: "_id",
                as: "businessInfo"
            }

            // Uncorrelated Subqueries
            // (supported as of MongoDB 3.6)
            // {
            //    from: "<collection to join>",
            //    let: { <var_1>: <expression>, …, <var_n>: <expression> },
            //    pipeline: [ <pipeline to execute on the collection to join> ],
            //    as: "<output array field>"
            // }
        },

        // Stage 2
        {
            $project: {
                businessId: 1,
                businessInfo: {
                    $arrayElemAt: ['$businessInfo', 0]
                }
            }
        },

        // Stage 3
        {
            $addFields: {
                "businessInfo.canClaim": {
                    $cond: {
                        if: {
                            $gte: [{
                                $indexOfArray: ["$businessInfo.capabilities", 'claiming']
                            }, 0]
                        },
                        then: true,
                        else: false
                    }
                }
            }
        },

    ]



);

- Rubin Porwal