将$ match答案从一个集合转发到Mongo中的另一个$ match [英] Forward $match answer from one collection to another $match in Mongo

问题描述

背景

我有一个mongo收藏夹标签:

I have a mongo collection tags:

...
{ "_id" : 901, "tagkey" : "color", "tagvalue" : "black" }
{ "_id" : 902, "tagkey" : "color", "tagvalue" : "white" }
{ "_id" : 903, "tagkey" : "store", "tagvalue" : "supercenter" }
{ "_id" : 904, "tagkey" : "store", "tagvalue" : "gas station" }
...

和另一个收藏集项目:

...
{ "_id" : 12, "itemname" : "truck", "tags" : [901] }
{ "_id" : 13, "itemname" : "ink", "tags" : [901, 903] }
{ "_id" : 14, "itemname" : "paper", "tags" : [902, 903] }
{ "_id" : 14, "itemname" : "gas", "tags" : [904] }
...

我正在尝试询问商店中的所有对象. 这意味着我想要一个列表的所有商品，这些商品的标签都带有一个键名:"store".因此，我将在其标签列表中输出所有包含903或904的项目. 因此，我应该获取包含墨水，纸张和气体的列表.

I am trying to ask for all the objects that are in a store. This means I want a list of all items that have a tag with a keyname: "store". So I am outputting all the items containing 903 or 904 in their tags list. So, I should get back a list containing ink, paper and gas.

问题

如何使用一个表的$match输出作为下一个查询的值?

How do I use the output from one table's $match as the value for my next query?

我最接近的猜测

db.tags.aggregate ( 
    {$match: {tagkey: "store"}}, # now I have a list of all the tag items
    {$match: {tags: {$elemMatch: {$in :[****]} } }} ) #Here I need to run the match query on a different collection and match the tags element to one of the _ids from the first match
)

更具体的问题

1. 如何获取第二个$match来引用其他集合
2. 如何获取该元素数组作为_id的输出 第一个$match

1. How do I get the second $match to refer to a different collection
2. How do I get that array of elements to be the _ids of the output of the first $match

推荐答案

您可以使用 $ ne 运算符

You can use $lookup with uncorrelated subqueries syntax to define your matching criteria and then check if there are any tags assigned to each item using $ne operator

db.items.aggregate([
    {
        $lookup: {
            from: "tags",
            let: { tags: "$tags" },
            pipeline: [
                {
                    $match: {
                        $expr: {
                                $and: [
                                { $in: [ "$_id", "$$tags" ] },
                                { $eq: [ "$tagkey", "store" ] }
                            ]
                        }
                    }
                }
            ],
            as: "tagDetails"
        }
    },
    {
        $match: {
            tagDetails: {
                $ne: []
            }
        }
    }
])

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