$out stage

According to the mongodb documentation

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

CodePudding user response：

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.