为MongoDB聚合中的字段分配增量编号 [英] Assign an incremental number to a field in a MongoDB aggregation

问题描述

我有一个MongoDB聚合，并且每个文档都有一个像这样的字段(groupNumber):

I have a MongoDB aggregation and each document has a field (groupNumber) like this:

我需要每个groupNumber的每个文档具有不同的编号(可以是1,2,3 ..递增)

I need that each groupNumber has a different number for each document (could be incremental 1,2,3..)

我找到的大多数解决方案都在使用查找" 像这样，但我认为不能在聚合中使用.

Most of the solutions that I had found are using "find" like this one , but I think that cannot be used in an aggregation.

提前谢谢.

推荐答案

您可以通过查找来完成此操作，但是我认为它不能很好地扩展.我认为应该以某种方式将其保留下来，也许 $ out 到另一个集合中，以使数字稳定，并且删除或插入文档不会改变任何其他组的数字.

You can do this with a lookup, but I don't think it will scale well. I would think this should be persisted in some way, perhaps $out to another collection, so that the numbers are stable and deleting or inserting a document won't change the numbers for any other group.

db.target.aggregate([
    {"$lookup":{
        "from":"target",
        "as":"looked",
        "let":{"srcId":"$_id"},
        "pipeline":[
          {"$match":{"$expr":{"$lte":["$_id","$$srcId"]}}},
          {"$group":{"_id":"null", "cnt":{"$sum":1}}}
        ]
    }},
    {"$addFields":{"groupNumber":{"$arrayElemAt":["$looked.cnt",0]}}},
    {"$project":{"looked":0}}
])

这篇关于为MongoDB聚合中的字段分配增量编号的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！