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mongodb - Mongodb分组并使用空数组推送

问题描述

当有group一个数组可能是empty. 集合可能是这样的：

{
    "_id" : "Contract_1",
    "ContactId" : "Contact_1",
    "Specifications" : [ ]
}

{
    "_id" : "Contract_2",
    "ContactId" : "Contact_2",
    "Specifications" : [ 
        {
            "Description" : "Descrizione1",
            "VehicleId" : "Vehicle_1",
            "Customizations" : [ 
                {
                    "Description" : "Random furniture",
                    "ContactId" : "Contact_5"
                }, 
                {
                    "Description" : "Random furniture 2",
                    "ContactId" : "Contact_3"
                }
            ]
        }, 
        {
            "Description" : "Descrizione2",
            "VehicleId" : "Vehicle_2",
            "Customizations" : [ 
                {
                    "Description" : "Random furniture",
                    "ContactId" : "Contact_5"
                }, 
                {
                    "Description" : "Random furniture 2",
                    "ContactId" : "Contact_3"
                }
            ]
        }
    ]
}

{
    "_id" : "Contract_3",
    "ContactId" : "Contact_25",
    "Specifications" : [ 
        {
            "Description" : "Descrizione1",
            "VehicleId" : "Vehicle_1",
            "Customizations" : []
        }, 
        {
            "Description" : "Descrizione2",
            "VehicleId" : "Vehicle_2",
            "Customizations" : []
        }
    ]
}

如您所见，有时Specifications可以为 null，也可以为Customizations. 这是我执行的查询：

db.getCollection("Contract").aggregate([
  { "$lookup": {
    "from": "Contact",
    "localField": "ContactId",
    "foreignField": "_id",
    "as": "Contact"
  }},
  { "$unwind": {"path":"$Contact", "preserveNullAndEmptyArrays":true }},
  { "$unwind": { "path": "$Specifications", "preserveNullAndEmptyArrays":true }},
  { "$lookup": {
    "from": "Vehicle",
    "localField": "Specifications.VehicleId",
    "foreignField": "_id",
    "as": "Specifications.Vehicle"
  }},
  { "$unwind": {"path": {"$Specifications.Vehicle","preserveNullAndEmptyArrays":true} },
  { "$unwind": {"path": {"$Specifications.Customizations","preserveNullAndEmptyArrays":true} },
  { "$lookup": {
    "from": "Contact",
    "localField": "Specifications.Customizations.ContactId",
    "foreignField": "_id",
    "as": "Specifications.Customizations.Contact"
  }},
  { "$unwind": {"path": {"$Specifications.Customizations.Contact","preserveNullAndEmptyArrays":true} },
  { "$group": {
    "_id": {
      "_id": "$_id",
      "Description": "$Specifications.Description"
    },
    "ContactId": { "$first": "$ContactId" },
    "Contact": { "$first": "$Contact" },
    "Specifications": {
      "$push": "$Specifications.Customizations"
    }
  }},
  { "$group": {
    "_id": "$_id._id",
    "ContactId": { "$first": "$ContactId" },
    "Contact": { "$first": "$Contact" },
    "Specifications": {
      "$push": {
        "Description": "$_id.Description",
        "Customizations": "$Specifications"
      }
    }
  }}
])
}},
   { "$group": {
    "_id": "$_id._id",
    "ContactId": { "$first": "$ContactId" },
    "Contact": { "$first": "$Contact" },
    "Specifications": {
      "$push": {
        "Description": "$_id.Description",
        "Customizations": "$Specifications"
      }
    }
  }}
])

一旦查询执行，当它执行 2$group时就会产生问题，因为对于第一个 whenpushing $Specifications.Customizations将创建一个内部包含空元素的数组。我想要的是 IfSpecifications是一个空数组，将保持不变而不在里面添加一个空元素。

标签： mongodbmongodb-queryaggregation-framework

解决方案

这是我可以看到嵌套数组的$unwindand的缺点之一。$group要摆脱这种情况，您需要再添加一个阶段$addFields来过滤掉空的嵌套数组。

将其添加到管道的末尾

{ "$addFields": {
  "Specifications": {
    "$filter": {
      "input": "$Specifications",
      "cond": { "$ne": ["$$this.Description", undefined] }
    }
  }
}}

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