$out and $merge for writing results in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $out stage do in a MongoDB aggregation pipeline?

The $out stage writes the results of the aggregation pipeline to a specified collection, replacing the existing data in that collection.

intermediate

How does $merge differ from $out in MongoDB?

$merge writes results to a collection but can merge with existing documents based on a specified key, allowing updates or inserts without replacing the whole collection.

intermediate

Can $out be used to write results to a different database than the source?

Yes, $out can write to a collection in a different database by specifying the database name along with the collection name.

beginner

What happens if the target collection does not exist when using $merge?

If the target collection does not exist, $merge creates it and inserts the aggregation results.

intermediate

Which stage would you use to update existing documents based on a matching field during aggregation: $out or $merge?

You would use $merge because it supports updating existing documents based on a matching field, while $out replaces the entire collection.

What does the $out stage do in MongoDB aggregation?

A Filters documents based on a condition

B Merges results with existing documents

C Writes aggregation results to a collection, replacing it

D Sorts documents in the pipeline

$out writes the aggregation results to a collection and replaces the existing data.

Which stage allows updating existing documents during aggregation?

A <code>$merge</code>

B <code>$out</code>

C <code>$match</code>

D <code>$group</code>

$merge can update existing documents based on a matching key.

If you want to write aggregation results to a new database, which stage can you use?

A <code>$merge</code> only

B <code>$out</code> only

C Both <code>$out</code> and <code>$merge</code>

D Neither <code>$out</code> nor <code>$merge</code>

$out supports writing to a collection in a different database by specifying the database name.

What happens if the target collection does not exist when using $merge?

A The collection is created and results are inserted

B Existing collection is dropped

C The pipeline stops without writing

D The operation fails

$merge creates the target collection if it does not exist.

Which stage replaces the entire target collection with aggregation results?

A <code>$lookup</code>

B <code>$merge</code>

C <code>$project</code>

D <code>$out</code>

$out replaces the entire target collection with the aggregation output.

Explain the main differences between $out and $merge stages in MongoDB aggregation.

Think about how each stage handles existing data in the target collection.

Describe a scenario where using $merge is better than $out.

Consider when you want to keep some existing data and only update parts.