$setField

The $setField operator is used to add, update, or remove fields in embedded documents. The operator allows for precise manipulation of document fields, which makes it useful for tasks such as updating nested fields, restructuring documents, or even removing fields entirely.

Syntax

{
  $setField: {
    field: <fieldName>,
    input: <expression>,
    value: <expression>
  }
}

Parameters

Parameter Description
<field> The document (object) to be transformed into an array of key-value pairs.
<input> The document or field being processed.
<value> The new value to assign to the field. If value is null, the field is removed.

Examples

Example 1: Updating a nested field

This query performs a conditional update on nested discount values inside promotion events for the document matching a specific _id.

db.stores.updateOne(
  { "_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4" },
  [
    {
      $set: {
        "store.promotionEvents": {
          $map: {
            input: "$store.promotionEvents",
            as: "event",
            in: {
              $setField: {
                field: "discounts",
                input: "$$event",
                value: {
                  $map: {
                    input: "$$event.discounts",
                    as: "discount",
                    in: {
                      $cond: {
                        if: { $eq: ["$$discount.categoryName", "Laptops"] },
                        then: {
                          categoryName: "$$discount.categoryName",
                          discountPercentage: 18
                        },
                        else: "$$discount"
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  ]
)

Example 2: Removing a field

Suppose you want to remove the totalStaff field from the staff object.

db.collection.updateOne(
  { "store.storeId": "12345" },
  [{
    $set: {
      "store.staff": {
        $setField: {
          field: "totalStaff",
          input: "$store.staff",
          value: null
        }
      }
    }
  }]
)

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