AWS DocumentDB 是否支持在单个查询中加入 3 个以上的集合?

问题描述

是否可以使用单个查询将 AWS DocumentDB 中的 3 个单独集合连接在一起(类似于此处的线程解决方案:How我可以在 MongoDB 聚合管道中执行嵌套的连接"(连接 3 个或更多集合)吗?

Is it possible to join 3 separate collections together in AWS DocumentDB with a single query (similar to the solution to the thread here: How can I perform nested "joins" (joining 3 or more collections) in a MongoDB aggregation pipeline?

我觉得可能不会，但我认为 AWS 不会直接出现并在任何地方明确表示.

I feel like it may not, but I don't think AWS comes right out and specifically says so anywhere.

开发人员指南 说:

Amazon DocumentDB 支持进行相等匹配的能力(对于例如，左外连接)但不支持不相关子查询.

如果我知道什么是不相关的子查询"，那会更有帮助.

That would be more helpful if I knew what an "uncorrelated subquery" was.

此外，DocumentDB 支持的 MongoDB API 列表 表示完全不支持 $let 变量运算符.他们所指的 $let 变量运算符是否与以下查询中使用的 $lookup 阶段中的 let 表达式完全相同将 3 个系列合并到一起?

Also, the list of MongoDB APIs that DocumentDB supports says that $let variable operator is outright not supported. Is the $let variable operator they're referring to exactly the same as the let expression within the $lookup stage that's used in the following query to join together 3 collections?

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"
    }
  }
])

在 AWS DocumentDB 中是否可以在单个 NoSQL 查询中加入 3 个以上的集合，如果不能，推荐/最有效的方法是什么?

Is joining 3+ collections within a single NoSQL query possible in AWS DocumentDB, and if not, what would be the recommended / most efficient way to do this?

推荐答案

在 DocumentDB 文档:

Amazon DocumentDB 支持进行相等匹配(例如，左外连接)的能力，但不支持不相关的子查询.

在 MongoDB 文档中:

相等匹配具有以下语法:

{
   $lookup:
     {
       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>
     }
}

不相关的子查询具有以下语法:

{
   $lookup:
     {
       from: <collection to join>,
       let: { <var_1>: <expression>, …, <var_n>: <expression> },
       pipeline: [ <pipeline to execute on the collection to join> ],
       as: <output array field>
     }
}

所以 DocumentDB 不支持第二种语法.

So the 2nd syntax is not supported in DocumentDB.

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