$out

$out

$out writes the pipeline's results into a collection. It's the stage that "materializes" your aggregation output so you can reuse it later.

$out must be last

$out can only be the final stage of a pipeline. It doesn't return documents to the caller - instead it saves them.

Example: materialize a summary

db.listingsAndReviews.aggregate([
  {
    $group: {
      _id: "$property_type",
      properties: {
        $push: {
          name: "$name",
          accommodates: "$accommodates",
          price: "$price"
        }
      }
    }
  },
  { $out: "properties_by_type" }
])

Stage 1 groups listings by property_type and uses $push to build an array of each property's details. Stage 2 ($out) writes the result into a new collection called properties_by_type.

Behavior

• Creates the target collection if it doesn't exist.
• Replaces the collection's contents if it does exist.
• Writes to a collection in the current database.

Why use $out?

• Cache the results of expensive aggregations.
• Export analysis results into a new collection.
• Build reporting tables you can query directly.

TL;DR

• $out writes pipeline results to a collection.
• It must be the last stage.
• Creates the collection or replaces its contents.