Community Day Trivia #26: In MongoDB 4.2, what aggregation stages can be used to save result documents to a collection?

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1) $merge stage

Writes the results of the [aggregation pipeline $merge operator must be the last stage in the pipeline.

The $merge stage:

  • Can output to a collection in the same or different database.
    • Starting in MongoDB 4.4, $merge can output to the same collection that is being aggregated. For more information, see Output to the Same Collection that is Being Aggregated.
  • Creates a new collection if the output collection does not already exist.
  • Can incorporate results (insert new documents, merge documents, replace documents, keep existing documents, fail the operation, process documents with a custom update pipeline) into an existing collection.
  • Can output to a sharded collection. Input collection can also be shared.

2) $out

Takes the documents returned by the aggregation pipeline and writes them to a specified collection. Starting in MongoDB 4.4, you can specify the output database.
The $out stage must be the last stage in the pipeline. The $out operator lets the aggregation framework return result sets of any size.

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