如何在 Mongodb 中使用 $lookup 将引用的文档作为嵌套文档获取？答案

【问题标题】：How can i get the referenced dcoument as nested document with $lookup in Mongodb?如何在 Mongodb 中使用 $lookup 将引用的文档作为嵌套文档获取？
【发布时间】：2020-05-11 06:42:17
【问题描述】：

我正在使用 mongodb 数据库并希望在 2 collections 上应用 $lookup 但有特定条件。我有一个名为 Company 的集合，像这样

new Schema({
name: String,
benefit: String,
benefitDesc: String,
company_url: String,
logoUrl: String,
coverUrl: String,
desc: String,
createdAt: String,
categoryId: { type: Schema.Types.ObjectId, ref: 'categories' },
})

还有一个类似这样的名为 Referrallinks 的集合

new Schema({
referral_link: String,
referral_code: String,
isLink: Number,
offer_name: String,
offer_desc: String,
user_email: String,
companyId: { type: Schema.Types.ObjectId, ref: 'companies' },
addedByAdmin: { type: Boolean, default: true },
number_of_clicks: Number,
referral_country: String,
link_status: String,
categoryId: { type: Schema.Types.ObjectId, ref: 'categories' },
number_of_clicks: { type: Number, default: 0 },
createdAt: String,
 updatedAt: String,
 userId: { type: Schema.Types.ObjectId, ref: 'users' }
})

现在当我应用这个$lookup 查询时

Company.aggregate([{
    $lookup: {
        from: "referrallinks",
        let: { company_id: "$_id" },
        pipeline: [{
                $match: {
                    $expr: {
                        $and: [
                            { $eq: ["$$company_id", "$companyId"] },
                            { $eq: ["$link_status", "Approved"] }
                        ]
                    }
                }
            },
            {
                $lookup: {
                    from: "categories",
                    localField: "categoryId",
                    foreignField: "_id",
                    as: "category"
                }
            }
        ],
        as: "referrals"
    }
}])

所以我在每个公司的referral 对象的key 中获取category 对象。但我希望它在 referral 这样的对象之外

[
 {name : '', benefit : '', benefiDesc : '', referrals : [], category : {}},
 {name : '', benefit : '', benefiDesc : '', referrals : [], category : {}},
 {name : '', benefit : '', benefiDesc : '', referrals : [], category : {}},
 .....
]

我该怎么做？

【问题讨论】：

标签： mongodb mongoose lookup

【解决方案1】：

您只需将嵌套的$lookup 移出第一个并作为单独的管道步骤运行：

Company.aggregate([
    {
        $lookup: {
            from: "referrallinks",
            let: { company_id: "$_id" },
            pipeline: [{
                $match: {
                    $expr: {
                        $and: [
                            { $eq: [ "$$company_id", "$companyId" ] },
                            { $eq: [ "$link_status", "Approved" ] }
                        ]
                    }
                }
            }],
            as: "referrals"
        }
    },
    {
        $lookup: {
            from: "categories",
            localField: "referrals.categoryId",
            foreignField: "_id",
            as: "categories"
        }
    }
])

请注意，referrals.categoryId 的计算结果为 ObjectId 值数组

Mongo Playground

【讨论】：