访问关系的属性——MongoDb答案

【问题标题】：Access the properties of the relationship -- MongoDb访问关系的属性——MongoDb
【发布时间】：2023-04-01 05:35:01
【问题描述】：

我有两个模型：

const walletSchema = mongoose.Schema({
  title: { type: String, required: true },
  description: { type: String, required: false},
  amountStart: { type: Number, required: true},
  mount: {type: Number, required: false},
  transfers: [{ type: Schema.Types.ObjectId, ref:'Transfers' }],
});

和

const transferSchema = mongoose.Schema({
  mount: { type: Number, required: true },
  category_id: { type: Schema.ObjectId, ref: "Category", required: false },
  description: { type: String, required: false},
  wallet_id: { type: mongoose.Schema.Types.ObjectId, ref:"Wallet", required: true}
});

我想要获取钱包的挂载（amountStart - 所有转账挂载）

但我不知道该怎么做，谢谢！

【问题讨论】：

标签： angular mongodb express mongoose mean

【解决方案1】：

最简单的方法是聚合。

walletSchema.aggregate([
 // finds your wallet
 {$match : {_id: <theIdOfYourWallet}},
 // splits your array into multiple aggregation documents 
 // (see this for more info: https://stackoverflow.com/questions/34967482/lookup-on-objectids-in-an-array)
 {$unwind: 'transfers'},
 // fetches each transfer document
 { $lookup: {
    from: "transfers",
    localField: "transfers",
    foreignField: "_id",
    as: "transferInformation"
  }},
 // sums the transfers
 { $group:
   {
    _id: { amountStart: $amountStart },
    // not sure if you can access the .mount field directly like this
    totalTransferMount: { $sum: $transferInformation.mount },
    }
 },
 // calculating the total mount of the wallet
 { $project: { walletMount: { $subtract: [ "$_id.amountStart", "$totalTransferMount" ] } } }
])

我没有测试过，但是如果你理解了每一步，你应该没问题。

【讨论】：

我用这个解决了：walletSchema.aggregate([ { "$match": { "_id":
这是正确的，甚至更优雅。从 Mongodb 3.2 开始，$sum 运算符也可以在项目中使用

【解决方案2】：

walletSchema.aggregate([
    { "$match": { "_id": <theIdOfYourWallet } },
    { $lookup: {
       from: "transfers",
       localField: "transfers",
       foreignField: "_id",
       as: "transferInformation"
     }},
     { $project: {walletMount: {$sum: "$transferInformation.mount"}, amountStart: "$amountStart", transfers:[{transfer:"$transferInformation"}]}}
   ])

【讨论】：