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MongoDB Interview Preparation

Solution: Merging Documents

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Query
// Picking Furniture products from products collection
db.products.aggregate([
  {
    $match: {
      category: "Furniture"
    }
  },
  {
    $merge: {
      into: "furniture",  // Adding matched products to a new collection
      whenMatched: "keepExisting",
      whenNotMatched: "insert"
    }
  }
]),
// Deleting Furniture products from the original products collection
db.products.deleteMany({
  category: "Furniture"
}),
// Fetch documents from the FurnitureProducts collection
db.furniture.find()

The explanation of the query is given below:

  • Line 2: This starts an aggregation pipeline on the products collection.

  • Lines 4–6: The $match stage filters documents where the category field is "Furniture". Only products under the Furniture category will move to the next stage.

  • Lines 9–13: The $merge stage writes the filtered documents into a new or existing collection called "FurnitureProducts".

    • whenMatched: "keepExisting": If a document with the same _id ...