MongoDB Aggregation $out

MongoDB Aggregation `$out` 📤 (Write Pipeline Results)

In MongoDB, the $out stage is used in an Aggregation Pipeline to write the final result into a collection instead of returning it to the screen.

👉 Think of $out as “save aggregation result to a collection”.

What does `$out` do?

Writes aggregation output to a new or existing collection
Replaces the target collection if it already exists
Must be the last stage in the pipeline
Useful for reports, backups, materialized views

Basic Syntax

📌 The output collection will be created automatically if it doesn’t exist.

1️⃣ Simple `$out` Example

Copy all students into a new collection

✅ Creates students_backup collection with all documents from students.

2️⃣ `$out` with `$match`

Save only BA students

3️⃣ `$out` with `$group` (Reports)

Create a report: total students per course

📌 Now course_report stores precomputed results.

4️⃣ `$out` with `$project`

Store cleaned output

5️⃣ Verify Output Collection

`$out` vs `$merge`

`$out`	`$merge`
Replaces entire collection	Inserts / updates selectively
Deletes old data	Preserves existing data
Simple use cases	Advanced sync use cases

📌 Use $merge when you don’t want full replacement.

SQL Comparison

SQL

MongoDB

Important Rules ⚠️

$out must be the last stage
Target collection is fully replaced
Requires write permission
Cannot output to the same collection you’re reading from

Common Mistakes ❌

Using $out in the middle of pipeline
Accidentally overwriting important collections
Expecting $out to append data

Best Practices 👍

Use $out for reporting & backups
Prefer $merge for incremental updates
Always double-check output collection name

MongoDB Aggregation $out