$out

The $out operator is used in a MongoDB aggregation pipeline to write the result of the aggregation to a specified collection. The $out stage must be the last stage in the pipeline, and it effectively replaces the existing collection with the new one containing the aggregated results. If the specified collection does not exist, MongoDB will create it.

Syntax

Here's the basic syntax of the $out operator:

{
  $out: "target_collection_name"
}

Example

Let's say you have a students collection with the following documents:

[
  { "_id": 1, "name": "Alice", "score": 90 },
  { "_id": 2, "name": "Bob", "score": 85 },
  { "_id": 3, "name": "Charlie", "score": 92 }
]

You want to create a new collection that contains only the students who scored more than 88. You can use the $out operator to achieve this:

db.students.aggregate([
  {
    $match: { score: { $gt: 88 } }
  },
  {
    $out: "high_scorers"
  }
]);

After running this aggregation, a new collection named high_scorers will be created with the following documents:

[
  { "_id": 1, "name": "Alice", "score": 90 },
  { "_id": 3, "name": "Charlie", "score": 92 }
]

Considerations

• The $out operator replaces the target collection entirely. Any existing data in the target collection will be lost.

• The $out stage must be the last stage in the aggregation pipeline.

• The operation is atomic; it provides a consistent snapshot of the data.

• Indexes from the source collection are not copied to the target collection. You'll need to recreate any indexes that you need on the new collection.

• The $out operator can also be used to write results to a different database by specifying a fully qualified namespace (database.collection).

Use Cases

• Data Archiving: You can use $out to archive data by writing older records to a separate collection.

• Reporting: Generate complex reports and store them in a separate collection for quick access.

• Data Transformation: Perform complex transformations on your data and store the results in a new collection.