具有3个级别的MongoDB嵌套查找 [英] MongoDB nested lookup with 3 levels

问题描述

我需要从数据库中以JSON检索整个单个对象层次结构.实际上，关于实现此结果的任何其他解决方案的建议将被高度推荐.我决定使用具有$ lookup支持的MongoDB.

I need to retrieve the entire single object hierarchy from the database as a JSON. Actually the proposal about any other solution to achive this result would be highly appriciated. I decided to use MongoDB with its $lookup support.

所以我有三个收藏夹:

聚会

{ "_id" : "2", "name" : "party2" }
{ "_id" : "5", "name" : "party5" }
{ "_id" : "4", "name" : "party4" }
{ "_id" : "1", "name" : "party1" }
{ "_id" : "3", "name" : "party3" }    

地址

{ "_id" : "a3", "street" : "Address3", "party_id" : "2" }
{ "_id" : "a6", "street" : "Address6", "party_id" : "5" }
{ "_id" : "a1", "street" : "Address1", "party_id" : "1" }
{ "_id" : "a5", "street" : "Address5", "party_id" : "5" }
{ "_id" : "a2", "street" : "Address2", "party_id" : "1" }
{ "_id" : "a4", "street" : "Address4", "party_id" : "3" }

addressComment

{ "_id" : "ac2", "address_id" : "a1", "comment" : "Comment2" }
{ "_id" : "ac1", "address_id" : "a1", "comment" : "Comment1" }
{ "_id" : "ac5", "address_id" : "a5", "comment" : "Comment6" }
{ "_id" : "ac4", "address_id" : "a3", "comment" : "Comment4" }
{ "_id" : "ac3", "address_id" : "a2", "comment" : "Comment3" }

我需要检索所有具有所有相应地址和地址注释的各方，作为记录的一部分.我的汇总:

I need to retrieve all parties with all corresponding addresses and address comments as part of the record. My aggregation:

db.party.aggregate([{
    $lookup: {
        from: "address",
        localField: "_id",
        foreignField: "party_id",
        as: "address"
    }
},
{
    $unwind: "$address"
},
{
    $lookup: {
        from: "addressComment",
        localField: "address._id",
        foreignField: "address_id",
        as: "address.addressComment"
    }
}])

结果很奇怪.某些记录还可以.但是缺少_id 4的Party(没有地址).另外，结果集中有两个Party _id 1(但地址不同):

The result is pretty weird. Some records are ok. But Party with _id 4 is missing (there is no address for it). Also there are two Party _id 1 in the result set (but with different addresses):

{
    "_id": "1",
    "name": "party1",
    "address": {
        "_id": "2",
        "street": "Address2",
        "party_id": "1",
        "addressComment": [{
            "_id": "3",
            "address_id": "2",
            "comment": "Comment3"
        }]
    }
}{
    "_id": "1",
    "name": "party1",
    "address": {
        "_id": "1",
        "street": "Address1",
        "party_id": "1",
        "addressComment": [{
            "_id": "1",
            "address_id": "1",
            "comment": "Comment1"
        },
        {
            "_id": "2",
            "address_id": "1",
            "comment": "Comment2"
        }]
    }
}{
    "_id": "3",
    "name": "party3",
    "address": {
        "_id": "4",
        "street": "Address4",
        "party_id": "3",
        "addressComment": []
    }
}{
    "_id": "5",
    "name": "party5",
    "address": {
        "_id": "5",
        "street": "Address5",
        "party_id": "5",
        "addressComment": [{
            "_id": "5",
            "address_id": "5",
            "comment": "Comment5"
        }]
    }
}{
    "_id": "2",
    "name": "party2",
    "address": {
        "_id": "3",
        "street": "Address3",
        "party_id": "2",
        "addressComment": [{
            "_id": "4",
            "address_id": "3",
            "comment": "Comment4"
        }]
    }
}

请帮助我.我对MongoDB相当陌生，但是我觉得它可以满足我的需求.

Please help me with this. I'm pretty new to the MongoDB but I feel it can do what I need from it.

推荐答案

麻烦"的原因是第二个汇总阶段-{ $unwind: "$address" }.它将删除具有_id: 4的一方的记录(因为您的地址数组为空，如您所提到的)，并为具有_id: 1和_id: 5的一方生成两条记录(因为它们每个都有两个地址).

The cause of your 'troubles' is the second aggregation stage - { $unwind: "$address" }. It removes record for party with _id: 4 (because its address array is empty, as you mention) and produces two records for parties _id: 1 and _id: 5 (because each of them has two addresses).

• 为防止删除无地址的参与者，您应该设置

• To prevent removing of parties without addresses you should set preserveNullAndEmptyArrays option of $unwind stage to true.

为防止当事方重复其不同地址，您应该添加 $group 聚集到您的管道中.另外，将 $project 阶段与

To prevent duplicating of parties for its different addresses you should add $group aggregation stage to your pipeline. Also, use $project stage with $filter operator to exclude empty address records in output.

db.party.aggregate([{
  $lookup: {
    from: "address",
    localField: "_id",
    foreignField: "party_id",
    as: "address"
  }
}, {
  $unwind: {
    path: "$address",
    preserveNullAndEmptyArrays: true
  }
}, {
  $lookup: {
    from: "addressComment",
    localField: "address._id",
    foreignField: "address_id",
    as: "address.addressComment",
  }
}, {
  $group: {
    _id : "$_id",
    name: { $first: "$name" },
    address: { $push: "$address" }
  }
}, {
  $project: {
    _id: 1,
    name: 1,
    address: {
      $filter: { input: "$address", as: "a", cond: { $ifNull: ["$$a._id", false] } }
    } 
  }
}]);

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