MongoDB聚合限制查询 [英] MongoDB Aggregation Limit Lookup

问题描述

我正在PyMongo中使用$ lookup成功加入"两个集合(此方法有效).我遇到的问题是，当我加入的第二个集合返回所有记录时，该大小可能超过BSON文档的大小.

I am using $lookup in PyMongo to successfully "join" two collections (this works). I am having a problem where the second collection I am joining in may exceed the BSON document size when it returns all of the records.

我希望使用$ limit来限制"match_docs"下允许加入的记录数，例如:每个obj_id的评论"中最多可以有100条记录:

I am looking to use $limit to limit the number of records that are allowed to join under "match_docs" eg: 100 records maximum from "comments" per obj_id:

db.indicators.aggregate([
  {
    "$lookup": {
      "from": "comments",
      "localField": "_id",
      "foreignField": "obj_id",
      "as": "match_docs"
    }
  }
])

我尝试了各种类型的$ limit，它似乎仅限制了整个结果的总数，而不仅限于联接.

I've tried various types of $limit, and it seems to only limit the total number of results overall, not just for the join.

推荐答案

从MongoDB 3.6开始，您可以使用不相关的子查询来限制查找:

Starting from MongoDB 3.6 you can use uncorrelated subqueries to limit the lookup:

db.indicators.aggregate([
{ $lookup: {
  from: 'comments',
  as: 'match_docs',
  let: { indicator_id: '$_id' },
  pipeline: [
    { $match: {
      $expr: { $eq: [ '$obj_id', '$$indicator_id' ] }
    } },
    // { $sort: { createdAt: 1 } }, // add sort if needed (for example, if you want first 100 comments by creation date)
    { $limit: 100 }
  ]
} }
])

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