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Merge Similar Collections with $unionWith

Learn to use the $unionWith operator in MongoDB to merge documents from multiple collections into a single aggregation pipeline, enabling combined analysis and reporting on similar datasets.

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The $unionWith operator in MongoDB is a powerful feature that allows us to combine the results of multiple collections into a single aggregation pipeline. This is especially useful when we have multiple collections with similar schemas and want to analyze or report on them as if they were a single dataset.

Setting up

Up until now, our database does not contain similar collections. Let's build up a use case for this. The tech team has decided to keep cancelled orders in a separate collection, instead of keeping them in the orders collection. The following query does this:

MongoDB
db.orders.aggregate([           // Picking cancelled orders from orders collection
  {
    $match: {
      status: "Cancelled"
    }
  },
  {
    $merge: {
      into: "cancelledOrders",   // Adding cancelled orders to a new collection
      whenMatched: "keepExisting",
      whenNotMatched: "insert"
    }
  }
]),
db.orders.deleteMany({          //  Deleting cancelled orders from orders collection
  status: "Cancelled"
}),
db.cancelledOrders.find().limit(5)    // Printing first 5 orders from the new collection

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