如何在 MongoDB 的一次往返中根据查询更新多个文档？

Question

Imagine you have a collection with documents as follows:

{
    "_id": "SOME_MONGO_ID",
    "rank": 1
}

And given an _id want to update increment all the documents with rank higher than it.

If I know the rank I can use UpdateMany to increment the ranks higher than it using a filter. However that would require two round trips one to get the rank and another to do the update many.

Is it possible to use a pipeline to lookup the rank and then do the update in one round trip?

I'm using the c# mongodb driver.

This needs to work inside a transaction so we can't use '$merge'

Answer 1

You can use $lookup with $merge . If you expect few and small documents to return, you can use '$match' first, but it has the disadvantage of temporarily putting all of these updated docs in one document:

db.collection.aggregate([
  {$match: {_id: "SOME_MONGO_ID"}},
  {$lookup: {
      from: "collection",
      as: "updated",
      let: {thRank: "$rank"},
      pipeline: [{$match: {$expr: {$gt: ["$rank", "$$thRank"]}}}]
    }
  },
  {$unwind: "$updated"},
  {$replaceRoot: {newRoot: "$updated"}},
  {$project: {rank: {$add: ["$rank", incVal]}}},
  {$merge: {into: "collection"}}
])

See how it works on the playground example

If you expect many documents (or large ones) to return, you can use $lookup first:

db.collection.aggregate([
  {$lookup: {
      from: "collection",
      as: "ref_id",
      let: {doc_id: ObjectId("62fb40acd762c3e1150e0134")},
      pipeline: [{$match: {$expr: {$eq: ["$_id", "$$doc_id"]}}}]
    }
  },
  {$match: {$expr: {$gt: ["$rank", {$first: "$ref_id.rank"}]}}},
  {$unset: "ref_id"},
  {$project: {rank: {$add: ["$rank", incVal]}}},
  {$merge: {into: "collection"}}
])

See how it works on the playground example - many docs