How to return Mongodb Aggregate pipeline docs to ONE document?-CodePudding

I know this has got to be simple, but for the life of me I can't seem to generate the correct final stage in my pipeline to get this working. Here are the documents output from a stage that I have in a mongo query:

{ "_id" : ObjectId("61435ceb233ce0118c1d93ec") }
{ "_id" : ObjectId("61435cf29598d31c17f0d839") }
{ "_id" : ObjectId("611e5cf953396d78985d222f") }
{ "_id" : ObjectId("61435cf773b8b06c848af83e") }
{ "_id" : ObjectId("61435cfd7ac204efa857e7ce") }
{ "_id" : ObjectId("611e5cf953396d78985d2237") }

I would like to get these documents into ONE single document with an array as such:

{ 
   "_id" : [
      ObjectId("61435ceb233ce0118c1d93ec"),
      ObjectId("61435cf29598d31c17f0d839"),
      ObjectId("611e5cf953396d78985d222f"),
      ObjectId("61435cf773b8b06c848af83e"),
      ObjectId("61435cfd7ac204efa857e7ce"),
      ObjectId("611e5cf953396d78985d2237")
   ]
}

My last stage in the pipeline is simply:

{
   $group:{_id:"$uniqueIds"}
}

I've tried everything from $push to $mergeObjects, but no matter what I do, it keeps returning the original 6 documents in some shape or form instead of ONE document. Any advice would be greatly appreciated! Thanks in advance.

CodePudding user response：

Test code here

Query

group by null, sees all collection as 1 group

db.collection.aggregate([
  {
    "$group": {
      "_id": null,
      "ids": {
        "$push": "$_id"
      }
    }
  },
  {
    "$unset": "_id"
  }
])