mongoose Aggregation returns empty array []?

Question

$out stage

According to the mongodb documentation

$out	Writes the resulting documents of the aggregation pipeline to a collection. To use the $out stage, it must be the last stage in the pipeline.

the $out stage is working perfectly and writing new collection, i'm using append function

.append({ $out: "aggr_out" })

to place it in the last stage.

The current behavior

Saving new collection to the database
Returning an empty array in the callback function

Expected behavior

Return the aggregate result

Versions of Node.js, Mongoose, and MongoDB

node.js v14.17.3 (LTS)
mongoose v5.13.13
mongoDB
- db version v4.0.23
- git version: 07c6611b38d2aacbdb1846b688db70b3273170fb
- build environment:
  - distmod: debian92
  - distarch: x86_64
  - target_arch: x86_64

Code aggregate

you want to wirte to a new collection and also return you the data? if you want this one simple solution would be to query the newly created collection. — Takis
– Takis, Commented Nov 25, 2021 at 12:27
@Takis_, yes this is simple, but i query the database twice ! — fetahokey
– fetahokey, Commented Nov 25, 2021 at 14:03
yes ok, but the data are tranfered only once to you, i don't think that you can out+return cursor with one query the same time. — Takis
– Takis, Commented Nov 25, 2021 at 14:28
@Takis_ , yes indeed, I'm querying the newly created collection. — fetahokey
– fetahokey, Commented Nov 25, 2021 at 14:30

prasad_ · Accepted Answer · 2021-11-25 12:26:12Z

1

It is the expected behavior; i.e., when you use the $out stage in an aggregation query the result of the query is written to the collection name specified in the $out. And, the query returns an empty cursor.

For example, take a collection with a document:

{ _id: 1, stuff: [ "bananas", "whales" ] }

The query:

var cur = db.collection.aggregate([
    { $out: "new_collection" }
])

When you run this query (in mongo shell), the console output is an empty cursor (though the new_collection is created).

cur.hasNext() returns false (indicates there are no result documents in the cursor).

Mongoose query returns an array (not a cursor). So, you see an empty array in the callback result value. This is to be expected.

Reference: db.collection.aggregate() - see the Returns section.

If the pipeline includes the $out operator, aggregate() returns an empty cursor.

answered Nov 25, 2021 at 12:26

prasad_

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2 Comments

prasad_ Over a year ago

The above query was run on a MongoDB v4.2.8 server and mongo shell client.

fetahokey Over a year ago

I didn't notice that the documentation mention this, thank you @prasad_

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