如何在 MongoDB 聚合管道中执行嵌套 "joins"（加入 3 个或更多集合）？

Question

假设我们在 MongoDB 中有 3 个假设的集合：customers、orders 和 orderItems。

每个客户有多个订单，每个订单有多个订单商品。

以下是这 3 个集合的一些样本数据：

客户

[
    {
        customer_id: 1,
        name: "Jim Smith",
        email: "jim.smith@example.com"
    },
    {
        customer_id: 2,
        name: "Bob Jones",
        email: "bob.jones@example.com"
    }
]

订单

[
    {
        order_id: 1,
        customer_id: 1
    },
    {
        order_id: 2,
        customer_id: 1
    }
]

订单商品

[
    {
        order_item_id: 1,
        name: "Foo",
        price: 4.99,
        order_id: 1
    },
    {
        order_item_id: 2,
        name: "Bar",
        price: 17.99,
        order_id: 1
    },
    {
        order_item_id: 3,
        name: "baz",
        price: 24.99,
        order_id: 2
    }
]

想要的结果

如何编写我的聚合管道以使返回的结果看起来像这样？

[
    {
        customer_id: 1,
        name: "Jim Smith",
        email: "jim.smith@example.com"
        orders: [
            {
                order_id: 1,
                items: [
                    {
                        name: "Foo",
                        price: 4.99
                    },
                    {
                        name: "Bar",
                        price: 17.99
                    }
                ]
            },
            {
                order_id: 2,
                items: [
                    {
                        name: "baz",
                        price: 24.99
                    }
                ]
            }
        ]
    },
    {
        customer_id: 2,
        name: "Bob Jones",
        email: "bob.jones@example.com"
        orders: []
    }
]

Answer 1

使用 lookup with pipeline、

进行嵌套查找

$lookup 与 orders collection,
- let，定义来自主 collection 的变量 customer_id，使用 $$ 像 $$customer_id，
- pipeline 可以像我们在根级管道中一样添加管道阶段
- $expr 每当我们匹配内部字段时它需要表达式匹配条件，所以 $$customer_id 是 parent collection 在 let 和 [= 中声明的字段21=]是childcollection's/currentcollection的字段
$lookup 与 orderitems collection

db.customers.aggregate([
  {
    $lookup: {
      from: "orders",
      let: { customer_id: "$customer_id" },
      pipeline: [
        { $match: { $expr: { $eq: ["$$customer_id", "$customer_id"] } } },
        {
          $lookup: {
            from: "orderitems",
            localField: "order_id",
            foreignField: "order_id",
            as: "items"
          }
        }
      ],
      as: "orders"
    }
  }
])

Playground

Tip:

Several joins considered as bad practice in NoSQL, I would suggest if you could add your order items in orders collection as array, you can save one join process for orderitems, see improved version in playground

如何在 MongoDB 聚合管道中执行嵌套 "joins"（加入 3 个或更多集合）？

How can I perform nested "joins" (joining 3 or more collections) in a MongoDB aggregation pipeline?

mongodb

nosql

aggregation-framework

nosql-aggregation

客户

订单

订单商品

想要的结果