$out and $merge: Writing Pipeline Results
Learners will direct aggregation output into a new or existing collection using $out and $merge for ETL and materialised views.
Writing Pipeline Results to Collections
By default, aggregation pipeline results are returned to the client as a cursor. Sometimes you want to persist the results into a MongoDB collection for later use as a materialised view, a reporting cache, or an ETL target. MongoDB provides two stages for this: $out, which replaces a target collection atomically, and $merge, which upserts or merges results into an existing collection.
The $out Stage
$out writes all pipeline output documents to a new or existing collection in an atomic operation. If the target collection exists, $out replaces it entirely with the new results—the old collection is dropped and the new one takes its place atomically. If it doesn't exist, MongoDB creates it. $out must be the last stage in the pipeline and returns nothing to the client.
// Materialise monthly sales summary into its own collection
db.orders.aggregate([
{ $match: { status: 'completed' } },
{ $group: {
_id: {
year: { $year: '$createdAt' },
month: { $month: '$createdAt' }
},
totalRevenue: { $sum: '$amount' },
orderCount: { $sum: 1 }
}},
{ $sort: { '_id.year': 1, '_id.month': 1 } },
{ $out: 'monthly_sales_summary' } // last stage, writes to collection
]);All lessons in this course
- $lookup: Joining Collections in the Pipeline
- $unwind: Deconstructing Array Fields
- $addFields, $replaceRoot, and $mergeObjects
- $out and $merge: Writing Pipeline Results