node.js - MongoDB - 如何从具有 ID 的对象数组中检索多个集合

Question

所以，我有一个包含以下集合的数据库。

(使用整数表示对象ID)

书籍:

books = [
  { _id: 1, title: "Harry Potter" },
  { _id: 2, title: "The maze runner" }
  ...
]

客户:

customers = [
  { _id: 1, name: "John" },
  { _id: 2, title: "Jane"}
  ...
]

建议:

recommendations = [
  {
    customerID: 1,
    recommendations: [
      { bookID: 1, likelihood: 0.9 },
      { bookID: 2, likelihood: 0.8 }
    ]
  },
  {
    customerID: 2,
    recommendations: [
      { bookID: 2, likelihood: 0.9 },
      { bookID: 1, likelihood: 0.8 }
    ]
  }
  ...
]

现在，当请求发送到我的服务器时，在 req.body 中包含 customerID，我想返回为该客户推荐的书籍，并附加可能性给他们。

即:

desiredResult = [
  {
    _id: 1,
    title: "Harry Potter",
    likelihood: 0.9
  },
  {
    _id: 2,
    title: "The maze Potter",
    likelihood: 0.8
  }
  ...
]

请问，实现此目的的 MongoDB 聚合查询是什么？

Answer 1

您可以使用下面的aggregation

db.recommendations.aggregate([
  { "$match": { "customerID": 1 }},
  { "$unwind": "$recommendations" },
  { "$lookup": {
    "from": "books",
    "localField": "recommendations.bookID",
    "foreignField": "_id",
    "as": "recomndedBook"
  }},
  { "$replaceRoot": {
    "newRoot": {
      "$mergeObjects": [
        { "$arrayElemAt": ["$recomndedBook", 0] },
        "$recommendations"
      ]
    }
  }}
])