Subset Pattern

• a book have N reviews
• only embed 3 reviews to the book

Code Summary: Subset Pattern To apply the Subset pattern to the documents in our bookstore app, we ran the following aggregation pipeline:

[
  {
    $unwind: {
      path: "$reviews"
    }
  },
  {
    $set: {
      "reviews.product_id": "$product_id"
    }
  },
  {
    $replaceRoot: {
      newRoot: "$reviews"
    }
  },
  {
    $out: "reviews"
  }
]

Next, we defined a clean up pipeline to ensure the reviews array in the book documents only includes three reviews:

[
   {
       reviews: {
           $slice: ["$reviews", 3]
       }
   }
];

Finally, we ran this pipeline for the books collection with the updateMany method:

db.books.updateMany({}, update_reviews_array_pipeline);