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With the help of this article, you will learn how to use $setthe operator to partially update objects in MongoDB so that the new object overlaps/merges with the existing object.

The $set operator replaces the value of a field with a given value. $setThe operator expression uses the following syntax.

{ $set: { <field1>: <value1>, ... } }

You would specify one in an embedded document or array using dot notation <field>.

MongoDB $set Operator

If the field does not exist, $setthe operator creates a field with the specified value as long as the new field does not violate any constraints.

For example, if you provide a dotted path to a field that does not exist, $setthe operator will generate embedded documents as needed to complete the dotted path.

Additionally, if you specify multiple field-value pairs, the $set operator updates or creates each field.

You can create the following product collections.

db={
  "products": [
    {
      _id: 100,
      quantity: 250,
      instock: true,
      reorder: false,
      details: {
        model: "PTI",
        make: "Breakout"
      },
      tags: [
        "jeans",
        "clothing"
      ],
      ratings: [
        {
          by: "CustomerIK",
          rating: 3
        }
      ]
    }
  ]
}

Setting the Top-Level Field

_idThe following operation uses the $set operator to update the values ​​of the details field, quantity field, and tags field for documents matching the condition equal to 100. The query for this is given below.

db.products.update(
   { _id: 100 },
   { $set:
      {
        quantity: 500,
        details: { model: "2600", make: "Outfitters" },
        tags: [ "coats", "outerwear", "clothing" ]
      }
   }
)

The above operation updates the content below.

1. The quantity value is 500.
2. Details field with new embedded document
3. Mark the fields with the new array

The results of the above query are shown in the screenshot below.

Setting fields in embedded documents

You would use dot notation to specify in an embedded document or array <field>. _idThe following operation updates the make field in the detail document for documents matching the condition equal to 100.

The query for this is given below.

db.products.update(
   { _id: 100 },
   { $set: { "details.make": "Breakout Kids" } }
)

The results of the above query are shown in the screenshot below.

Setting an element in an array

The following operation modifies the value in the second element of the tags field (array index 1) and the ratings field in the first element of the ratings array (array index 0) for documents that satisfy the condition _id equals 100.

The query for this is given below.

db.products.update(
   { _id: 100 },
   { $set:
      {
        "tags.1": "wind breaker",
        "ratings.0.rating": 2
      }
   }
)

The results of the above query are shown in the screenshot below.

$set (aggregation)

$set (aggregation) adds new fields to a document. $set outputs a document containing all existing fields from the input document plus the newly added fields.

$set(aggregate) has the following syntax.

{ $set: { <newField>: <expression>, ... } }

$set is a command that adds new fields to an existing document. Users can include one or more $set stages in an aggregation operation.

Use dot notation to add a field or fields to an embedded document (including documents in an array). Use $concatArraysto add elements to an existing array field using $set.

Using two $set stages

Create a sample grades collection using the following content.

db={
  "grades": [
    {
      _id: 1,
      student: "Ali",
      homework: [10, 8, 9],
      quiz: [9, 8],
      extraCredit: 3
    },
    {
      _id: 2,
      student: "Manahil",
      homework: [3, 8, 5],
      quiz: [7, 9],
      extraCredit: 6
    }
  ]
}

In the following process $setthree new fields are added to the output document using two stages. The query for this is given below.

db.grades.aggregate( [
   {
     $set: {
        totalHomework: { $sum: "$homework" },
        totalQuiz: { $sum: "$quiz" }
     }
   },
   {
     $set: {
        totalScore: { $add: [ "$totalHomework", "$totalQuiz", "$extraCredit" ] } }
   }
] )

The operation returns the following document as shown in the following screenshot.

Adding fields to embedded documents

Create a sample collection transport with the following content.

db={
  "transport": [
    {
      _id: 1,
      type: "sedan",
      specs: { doors: 4, wheels: 4}
    },
    {
      _id: 2,
      type: "motorbike",
      specs: {doors: 0, wheels: 2}
    },
    {
      _id: 3,
      type: "jet ski"
    }
  ]
}

The following aggregation operation creates a new field fuel_type in the embedded document specification. The query for this is given below.

db.transport.aggregate( [
   { $set: { "specs.fuel_type": "super" } }
] )

The operation returns the following results, as shown in the following screenshot.

Overwrite existing fields

When a $set operation specifies an existing field name, the original field is replaced. Use the following items to make a sample set of pets.

db={
  "pets": [
    {
      _id: 1,
      dogs: 10,
      cats: 5
    }
  ]
}

The subsequent $set operation overwrites the cats field. The query for this is given below.

db.pets.aggregate( [
  { $set: { "cats": 20 } }
] )

The operation returns the following document as shown in the following screenshot.

Adding elements to an array

Create a sample grades collection using the following records.

db={
  "grades": [
    {
      _id: 1,
      student: "Ali",
      homework: [10, 8, 9],
      quiz: [9, 8],
      extraCredit: 3
    },
    {
      _id: 2,
      student: "Manahil",
      homework: [3, 8, 5],
      quiz: [7, 9],
      extraCredit: 6
    }
  ]
}

Users can use the $set operator with the $concatArrays expression. This will add an element to an existing array field.

The following operation uses $set to replace the homework field with a new array whose elements are the concatenation of the current homework array and another array containing the new grades [7]. The query for this is given below.

db.grades.aggregate([
   { $match: { _id: 2 } },
   { $set: { homework: { $concatArrays: [ "$homework", [ 7 ] ] } } }
])

The operation returns the following document as shown in the following screenshot.

In this article, you learned how to use the $set operator to partially update objects in MongoDB so that the new object overlaps/merges with the existing object.