MongoDB Aggregation $out

MongoDB Tutorial

 MongoDB Aggregation $out

In MongoDB, $out is an aggregation stage used to write the result of an aggregation pipeline into a new collection.

 Think of $out as:

“Save aggregation result permanently”

 What is $out?

  • $out takes the final result of an aggregation pipeline

  • Stores it in a collection

  • The target collection can be:

    • Newly created

    • Or overwritten if it already exists

1️⃣ Basic Syntax


 

 $out must be the LAST stage in the pipeline.

2️⃣ Simple Example (Most Common)

Scenario

You have a users collection and want to store only users from Delhi into a new collection.

Source Collection: users


 

 Result:

  • Creates a new collection: delhiUsers

  • Stores filtered documents permanently

3️⃣ Verify Output Collection


 

4️⃣ $out with Grouping (Interview Favorite)

Scenario

Count users by city and store result.


 

Output Collection: usersByCity


 

5️⃣ $out Overwrites Existing Collection

If the output collection already exists:

$out: "usersByCity"

 MongoDB drops and recreates it.

📌 Important Interview Point
❌ Existing data is lost
✔ Indexes are removed
✔ Only aggregation result remains

6️⃣ $out vs $merge (Very Important)

Feature$out$merge
Overwrites collection✅ Yes❌ Optional
Inserts new data❌ No✅ Yes
Updates existing docs❌ No✅ Yes
Safer for production❌✅

$merge is preferred in production systems.

7️⃣ When to Use $out?

✔ Creating summary collections
✔ Data transformation pipelines
✔ Reporting / analytics
✔ Backup of processed data
✔ Exam & interview questions

8️⃣ Restrictions & Rules

✔ $out must be last stage
✔ Cannot be used inside transactions
✔ Requires write permission
✔ Drops target collection if exists

 Key Points (Very Important for Exams & Interviews)

✔ $out writes aggregation result to a collection
✔ Creates new collection automatically
✔ Overwrites existing collection
✔ Must be last pipeline stage
✔ Not recommended for live production updates

 Common Interview Questions

Q1. What does $out do in MongoDB?
👉 Saves aggregation results into a collection

Q2. Can $out update existing documents?
👉 ❌ No

Q3. What happens if output collection exists?
👉 It is dropped and recreated

Q4. Which is safer: $out or $merge?
👉 $merge

 Summary


 

 That’s everything you need to know about MongoDB Aggregation $out.

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