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Operators

$setField (aggregation)

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  • Definition
  • Syntax
  • Behavior
  • Examples

Definition

$setField

New in version 5.0.

Adds, updates, or removes a specified field in a document.

You can use $setField to add, update, or remove fields with names that contain periods (.) or start with dollar signs ($).

Tip

Use $getField to retrieve the values of fields that contain dollar signs ($) or periods (.) that you add or update with $setField.

Syntax

$setField has the following syntax:

{
  $setField: {
    field: <String>,
    input: <Object>,
    value: <Expression>
  }
}

You must provide the following fields:

Field
Type
Description

field

String

Field in the input object that you want to add, update, or remove. field can be any valid expression that resolves to a string constant.

input

Object

Document that contains the field that you want to add or update. input must resolve to an object, missing, null, or undefined.

value

Expression

The value that you want to assign to field. value can be any valid expression.

Set to $$REMOVE to remove field from the input document.

Behavior

  • If input evaluates to missing, undefined, or null, $setField returns null and does not update input.

  • If input evaluates to anything other than an object, missing, undefined, or null, $setField returns an error.

  • If field resolves to anything other than a string constant, $setField returns an error.

  • If field doesn't exist in input, $setField adds it.

  • $setField doesn't implicitly traverse objects or arrays. For example, $setField evaluates a field value of "a.b.c" as a top-level field "a.b.c" instead of as a nested field, { "a": { "b": { "c": } } }.

  • $unsetField is an alias for $setField with an input value of $$REMOVE. The following expressions are equivalent:

    {
       $setField: {
          field: <field name>,
          input: “$$ROOT”,
          value: "$$REMOVE"
       }
    }
    {
       $unsetField: {
          field: <field name>,
          input: “$$ROOT”
       }
    }

Tip

See also:

Considerations for field names

Examples

Add Fields that Contain Periods (.)

Consider an inventory collection with the following documents:

db.inventory.insertMany( [
   { "_id" : 1, "item" : "sweatshirt", price: 45.99, qty: 300 }
   { "_id" : 2, "item" : "winter coat", price: 499.99, qty: 200 }
   { "_id" : 3, "item" : "sun dress", price: 199.99, qty: 250 }
   { "_id" : 4, "item" : "leather boots", price: 249.99, qty: 300 }
   { "_id" : 5, "item" : "bow tie", price: 9.99, qty: 180 }
] )

The following operation uses the $replaceWith pipeline stage and the $setField operator to add a new field to each document, "price.usd". The value of "price.usd" will equal the value of "price" in each document. Finally, the operation uses the $unset pipeline stage to remove the "price" field.

db.inventory.aggregate( [
   { $replaceWith: {
        $setField: {
           field: "price.usd",
           input: "$$ROOT",
           value: "$price"
   } } },
   { $unset: "price" }
] )

The operation returns the following results:

[
  { _id: 1, item: 'sweatshirt', qty: 300, 'price.usd': 45.99 },
  { _id: 2, item: 'winter coat', qty: 200, 'price.usd': 499.99 },
  { _id: 3, item: 'sun dress', qty: 250, 'price.usd': 199.99 },
  { _id: 4, item: 'leather boots', qty: 300, 'price.usd': 249.99 },
  { _id: 5, item: 'bow tie', qty: 180, 'price.usd': 9.99 }
]

Add Fields that Start with a Dollar Sign ($)

Consider an inventory collection with the following documents:

db.inventory.insertMany( [
   { "_id" : 1, "item" : "sweatshirt", price: 45.99, qty: 300 }
   { "_id" : 2, "item" : "winter coat", price: 499.99, qty: 200 }
   { "_id" : 3, "item" : "sun dress", price: 199.99, qty: 250 }
   { "_id" : 4, "item" : "leather boots", price: 249.99, qty: 300 }
   { "_id" : 5, "item" : "bow tie", price: 9.99, qty: 180 }
] )

The following operation uses the $replaceWith pipeline stage and the $setField and $literal operators to add a new field to each document, "$price". The value of "$price" will equal the value of "price" in each document. Finally, the operation uses the $unset pipeline stage to remove the "price" field.

db.inventory.aggregate( [
   { $replaceWith: {
        $setField: {
           field: { $literal: "$price" },
           input: "$$ROOT",
           value: "$price"
   } } },
   { $unset: "price" }
] )

The operation returns the following results:

[
  { _id: 1, item: 'sweatshirt', qty: 300, '$price': 45.99 },
  { _id: 2, item: 'winter coat', qty: 200, '$price': 499.99 },
  { _id: 3, item: 'sun dress', qty: 250, '$price': 199.99 },
  { _id: 4, item: 'leather boots', qty: 300, '$price': 249.99 },
  { _id: 5, item: 'bow tie', qty: 180, '$price': 9.99 }
]

Update Fields that Contain Periods (.)

Consider an inventory collection with the following documents:

db.inventory.insertMany( [
   { _id: 1, item: 'sweatshirt', qty: 300, 'price.usd': 45.99 },
   { _id: 2, item: 'winter coat', qty: 200, 'price.usd': 499.99 },
   { _id: 3, item: 'sun dress', qty: 250, 'price.usd': 199.99 },
   { _id: 4, item: 'leather boots', qty: 300, 'price.usd': 249.99 },
   { _id: 5, item: 'bow tie', qty: 180, 'price.usd': 9.99 }
] )

The following operation uses the $match pipeline stage to find a specific document and the $replaceWith pipeline stage and the $setField operator to update the "price.usd" field in the matching document:

db.inventory.aggregate( [
   { $match: { _id: 1 } },
   { $replaceWith: {
        $setField: {
           field: "price.usd",
           input: "$$ROOT",
           value: 49.99
    } } }
] )

The operation returns the following results:

[
  { _id: 1, item: 'sweatshirt', qty: 300, 'price.usd': 49.99 }
]

Update Fields that Start with a Dollar Sign ($)

Consider an inventory collection with the following documents:

db.inventory.insertMany([
   { _id: 1, item: 'sweatshirt', qty: 300, '$price': 45.99 },
   { _id: 2, item: 'winter coat', qty: 200, '$price': 499.99 },
   { _id: 3, item: 'sun dress', qty: 250, '$price': 199.99 },
   { _id: 4, item: 'leather boots', qty: 300, '$price': 249.99 },
   { _id: 5, item: 'bow tie', qty: 180, '$price': 9.99 }
] )

The following operation uses the $match pipeline stage to find a specific document and the $replaceWith pipeline stage and the $setField and $literal operators to update the "$price" field in the matching document:

db.inventory.aggregate( [
   { $match: { _id: 1 } },
   { $replaceWith: {
        $setField: {
           field: { $literal: "$price" },
           input: "$$ROOT",
           value: 49.99
   } } }
] )

The operation returns the following results:

[
  { _id: 1, item: 'sweatshirt', qty: 300, '$price': 49.99 }
]

Remove Fields that Contain Periods (.)

Consider an inventory collection with the following documents:

db.inventory.insertMany([
   { _id: 1, item: 'sweatshirt', qty: 300, 'price.usd': 45.99 },
   { _id: 2, item: 'winter coat', qty: 200, 'price.usd': 499.99 },
   { _id: 3, item: 'sun dress', qty: 250, 'price.usd': 199.99 },
   { _id: 4, item: 'leather boots', qty: 300, 'price.usd': 249.99 },
   { _id: 5, item: 'bow tie', qty: 180, 'price.usd': 9.99 }
] )

The following operation uses the $replaceWith pipeline stage and the $setField operator and $$REMOVE to remove the "price.usd" field from each document:

db.inventory.aggregate( [
   { $replaceWith:  {
        $setField: {
           field: "price.usd",
           input: "$$ROOT",
           value: "$$REMOVE"
   } } }
] )

The operation returns the following results:

[
  { _id: 1, item: 'sweatshirt', qty: 300 },
  { _id: 2, item: 'winter coat', qty: 200 },
  { _id: 3, item: 'sun dress', qty: 250 },
  { _id: 4, item: 'leather boots', qty: 300 },
  { _id: 5, item: 'bow tie', qty: 180 }
]

A similar query written using the $unsetField alias returns the same results:

db.inventory.aggregate( [
   { $replaceWith:  {
        $unsetField: {
           field: "price.usd",
           input: "$$ROOT"
   } } }
] )

Remove Fields that Start with a Dollar Sign ($)

Consider an inventory collection with the following documents:

db.inventory.insertMany( [
   { _id: 1, item: 'sweatshirt', qty: 300, '$price': 45.99 },
   { _id: 2, item: 'winter coat', qty: 200, '$price': 499.99 },
   { _id: 3, item: 'sun dress', qty: 250, '$price': 199.99 },
   { _id: 4, item: 'leather boots', qty: 300, '$price': 249.99 },
   { _id: 5, item: 'bow tie', qty: 180, '$price': 9.99 }
} )

The following operation uses the $replaceWith pipeline stage, the $setField and $literal operators, and $$REMOVE to remove the "$price" field from each document:

db.inventory.aggregate( [
   { $replaceWith: {
        $setField: {
           field: { $literal: "$price" },
           input: "$$ROOT",
           value: "$$REMOVE"
   } } }
] )

The operation returns the following results:

[
  { _id: 1, item: 'sweatshirt', qty: 300 },
  { _id: 2, item: 'winter coat', qty: 200 },
  { _id: 3, item: 'sun dress', qty: 250 },
  { _id: 4, item: 'leather boots', qty: 300 },
  { _id: 5, item: 'bow tie', qty: 180 }
]

A similar query written using the $unsetField alias returns the same results:

db.inventory.aggregate( [
  { $replaceWith: {
       $unsetField: {
          field: { $literal: "$price" },
          input: "$$ROOT"
  } } }
] )

Tip

See also:

$unsetField

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