If only a collection name is provided to the $out stage, what database is used?

When only a collection name is provided to the $out stage, the database that is utilized is the same one in which the aggregation operation is being performed. This behavior aligns with how MongoDB handles the context of operations; the $out stage is designed to write results directly into a specific collection within the same database as the aggregation. Therefore, if a user specifies just the collection name, there is an implicit understanding that the results should be directed to that collection within the current working database.

This approach helps maintain clarity and consistency in operations, ensuring that data flows seamlessly within the correct context without requiring further specification of the database every time an output collection is created. Since there is no indication of switching databases during the operation, the aggregation's database context remains fixed, making this option the most logical and naturally fitting choice.