mongodb - 没有本地的聚合查找

Question

So I have two ObjectId s that are in different collections, which I would like to be merged as an output. So it's like using the $lookup function without any local fields. Without an aggregate I could simply do two .findOne , but that would require two API calls (which would create two connections). Is it possible to achieve it in one?

So without the aggregation it would just look something like this:

let main_document =
    db.findOne({
        _id: ObjectId(first)
    })

let subdocument =
    db.findOne({
        _id: ObjectId(second)
    })

main_document.subdocument = subdocument

return main_document

Answer 1

You can use below aggregation

You don't need to specify the localField in the newer $lookup syntax. Just use $match inside the sub pipeline of $lookup aggregation

db.main_document.aggregate([
  { "$match": { "_id": ObjectId(first) }},
  { "$lookup": {
    "from": "subdocument",
    "pipeline": [
      { "$match": { "_id": ObjectId(second) }},
    ],
    "as": "subdocument"
  }},
  { "$unwind": "$subdocument" }
])