$unwind operator - ( MongoDB Misc )

The $unwind operator is used in MongoDB to transform an array into a stream of documents, where each document contains a single value from the original array. In this page, we will discuss the syntax, example, output, explanation, use, important points, and summary of the $unwind operator in MongoDB.

Syntax

The $unwind operator has the following syntax:

db.collection.aggregate({
   $unwind: {
      path: "arrayField"
   }
})

Here, arrayField is the name of the array field to unwind.

Example

Consider a collection named orders with the following documents:

{
   "_id": 1,
   "name": "Order 1",
   "items": [
      {
         "name": "Item 1",
         "price": 10
      },
      {
         "name": "Item 2",
         "price": 20
      }
   ]
},
{
   "_id": 2,
   "name": "Order 2",
   "items": [
      {
         "name": "Item 3",
         "price": 30
      },
      {
         "name": "Item 4",
         "price": 40
      }
   ]
}

In this example, items is an array that we want to unwind. The following query uses the $unwind operator to unwind the items array:

db.orders.aggregate({ $unwind: "$items" })

This will transform each document into two documents, one for each item in the items array:

{
   "_id": 1,
   "name": "Order 1",
   "items": {
      "name": "Item 1",
      "price": 10
   }
},
{
   "_id": 1,
   "name": "Order 1",
   "items": {
      "name": "Item 2",
      "price": 20
   }
},
{
   "_id": 2,
   "name": "Order 2",
   "items": {
      "name": "Item 3",
      "price": 30
   }
},
{
   "_id": 2,
   "name": "Order 2",
   "items": {
      "name": "Item 4",
      "price": 40
   }
}

Output

The output of the $unwind operator is a stream of documents, where each document contains a single value from the original array.

Explanation

The $unwind operator is used to transform an array field into a stream of documents. When the $unwind operator is applied to an array field, it creates a new document for each element in the array field, containing the entire document’s fields but only one field from the array.

Use

The $unwind operator is useful when you want to perform aggregation operations on the elements of an array field. By unwinding the array, each element becomes a separate document, which can be filtered, sorted, or grouped as required for aggregation operations.

Important Points

• The $unwind operator is used to transform an array into a stream of documents.
• The $unwind operator works only on array fields.
• The $unwind operator creates a new document for each element in the array field.

Summary

In this page, we discussed the $unwind operator in MongoDB. We covered the syntax, example, output, explanation, use, important points, and summary of the $unwind operator. By using the $unwind operator, you can transform an array field into a stream of documents and perform aggregation operations on each element of the array.