Node.js mongodb查找检索没有子对象的父对象

我有一个关于mongodb查询中的问题的问题： 我无法检索没有儿童组的调查 有关更多信息，请查看以下链接：

---

您的查找没有问题。但是，每当您使用空数组字段展开文档时，请展开并完成删除该文档。这就是您无法获得结果的原因

您的问题可以通过将PreserveNullAndEmptyArray设置为null来解决

{
  $unwind:
    {
      path: <field path>, // add your unbind doc from lookup pipeline.
      preserveNullAndEmptyArrays: true
    }
}

{
$REWIND：
{
路径：，//从查找管道添加取消绑定的文档。
preserveNullAndEmptyArrays：true
}
}

是的，你救了我一天，谢谢你我更新了查询，一切正常

Survey.aggregate([
                    { "$lookup": {
                    "from": "groups",
                    "localField": "_id",
                    "foreignField": "survey",
                    "as": "groups"
                }},
                 { "$unwind": {
                     path: "$groups",
                     preserveNullAndEmptyArrays: true
                 } },
                    { "$lookup": {
                    "from": "questions",
                    "localField": "groups._id",
                    "foreignField": "group",
                    "as": "groups.questions"
                }},
                    { "$unwind": {
                        path: "$groups.questions",
                        preserveNullAndEmptyArrays: true
                    } },
                    { "$lookup": {
                    "from": "response",
                    "localField": "groups.questions._id",
                    "foreignField": "question",
                    "as": "groups.questions.responses"
                }},
                { "$group": {
                    "_id": {
                        "_id": "$_id",
                        "company": "$company",
                        "created_date": "$created_date",
                        "enabled": "$enabled",
                        "name": "$name",
                        "groups": {
                            "_id": "$groups._id",
                            "name": "$groups.name",
                            "order": "$groups.order",
                            "created_date": "$groups.created_date",
                            "survey": "$groups.survey"
                        }
                    },
                    "questions": { "$push": "$groups.questions" }
                }},
                { "$sort": { "_id": 1 } },
                { "$group": {
                    "_id": "$_id._id",
                    "company": { "$first": "$_id.company" },
                    "created_date": { "$first": "$_id.created_date" },
                    "enabled": { "$first": "$_id.enabled" },
                    "name": { "$first": "$_id.name" },
                    "groups": {
                        "$push": {
                            "_id": "$_id.groups._id",
                            "name": "$_id.groups.name",
                            "order": "$_id.groups.order",
                            "created_date": "$_id.groups.created_date",
                            "survey": "$_id.groups.survey",
                            "questions": "$questions"
                        }
                    }
                }},
                { "$sort": { "_id": 1 } }
            ]

---

很高兴知道。伟大的。

Survey.aggregate([
                    { "$lookup": {
                    "from": "groups",
                    "localField": "_id",
                    "foreignField": "survey",
                    "as": "groups"
                }},
                 { "$unwind": {
                     path: "$groups",
                     preserveNullAndEmptyArrays: true
                 } },
                    { "$lookup": {
                    "from": "questions",
                    "localField": "groups._id",
                    "foreignField": "group",
                    "as": "groups.questions"
                }},
                    { "$unwind": {
                        path: "$groups.questions",
                        preserveNullAndEmptyArrays: true
                    } },
                    { "$lookup": {
                    "from": "response",
                    "localField": "groups.questions._id",
                    "foreignField": "question",
                    "as": "groups.questions.responses"
                }},
                { "$group": {
                    "_id": {
                        "_id": "$_id",
                        "company": "$company",
                        "created_date": "$created_date",
                        "enabled": "$enabled",
                        "name": "$name",
                        "groups": {
                            "_id": "$groups._id",
                            "name": "$groups.name",
                            "order": "$groups.order",
                            "created_date": "$groups.created_date",
                            "survey": "$groups.survey"
                        }
                    },
                    "questions": { "$push": "$groups.questions" }
                }},
                { "$sort": { "_id": 1 } },
                { "$group": {
                    "_id": "$_id._id",
                    "company": { "$first": "$_id.company" },
                    "created_date": { "$first": "$_id.created_date" },
                    "enabled": { "$first": "$_id.enabled" },
                    "name": { "$first": "$_id.name" },
                    "groups": {
                        "$push": {
                            "_id": "$_id.groups._id",
                            "name": "$_id.groups.name",
                            "order": "$_id.groups.order",
                            "created_date": "$_id.groups.created_date",
                            "survey": "$_id.groups.survey",
                            "questions": "$questions"
                        }
                    }
                }},
                { "$sort": { "_id": 1 } }
            ]