来自可调整的国外集合的MongoDB聚合

问题描述

我的订单集合中的文档具有_client键，这是一个ObjectId引用另一个集合中的另一个实体.该集合可能是组织，并且可能是用户-我的意思是-这是变量集合.我想告诉Mongo查找是否在两个集合中都找到了_client id.

My documents in the orders collection has _client key, which is an ObjectId references to another entity in another collection.The collection could be organization and could be users - I mean - it's variable collection.I want to tell Mongo to lookup if the _client id is found in both collections.

{
    $lookup: {
      from: "users", // could be "organizations" 
      let: { "client": "$_client" }, // could be "_organization"
      pipeline: [
        { $match: { $expr: { $eq: ["$_id", "$$client"] }}},
      ],
      as: "client"
    }
  },
  {
    $unwind: "$client"
  },

我尝试设置两次查找，一次用于 _client ，一次用于 _organization ，但是当其中之一丢失时，我没有得到任何结果全部.

I have tried to just set up two look ups, once for _client and one for _organization however when there one of them is missing, I just got no results at all.

推荐答案

$unwind 过滤掉数组为空且不包含任何元素的文档.

$unwind filtered out the documents where arrays are empty and do not contain any element.

因此，您必须使用 preserveNullAndEmptyArrays 并将其设置为true

So, You have to use preserveNullAndEmptyArrays and set it to true

{ "$unwind": { "path": "$client", "preserveNullAndEmptyArrays": true }}

，与organizations

{ "$unwind": { "path": "$organization", "preserveNullAndEmptyArrays": true }}

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