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$unsetField (expression operator)

Definition

$unsetField

New in version 5.0.

Removes a specified field in a document.

You can use $unsetField to remove fields with names that contain periods (.) or that start with dollar signs ($).

$unsetField is an alias for $setField using $$REMOVE to remove fields.

Syntax

$unsetField has the following syntax:

{   $unsetField: {      field: <String>,      input: <Object>,   }}

You must provide the following fields:

Field
Type
Description

field

String

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

input

Object

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

Behavior

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

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

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

  • $unsetField doesn't implicitly traverse objects or arrays. For example, $unsetField 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": } } }.

Examples

Remove Fields that Contain Periods (.)

Consider the inventory collection:

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 } ] )

Use the $replaceWith pipeline stage and the $unsetField operator to remove the "price.usd" field from each document:

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

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 }]

Remove Fields that Start with a Dollar Sign ($)

Consider the inventory collection:

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 } ] )

Use the $replaceWith pipeline stage with the $unsetField and $literal operators to remove the "$price" field from each document:

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

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 }]

Remove A Subfield

Consider the inventory collection:

db.inventory.insertMany( [   { _id: 1, item: "sweatshirt", qty: 300, "price": {"usd":45.99, "euro": 38.77 } },   { _id: 2, item: "winter coat", qty: 200, "price": { "usd": 499.99, "euro": 420.51 } },   { _id: 3, item: "sun dress", qty: 250, "price": { "usd": 199.99, "euro": 167.70 } },   { _id: 4, item: "leather boots", qty: 300, "price": { "usd": 249.99, "euro": 210.68 } },   { _id: 5, item: "bow tie", qty: 180, "price": { "usd": 9.99, "euro": 8.42 } } ] )

The "price" field contains a document with two subfields, "usd" and "euro". You cannot use "price.euro" to identify and remove "euro" because MongoDB parses "price.euro" as a top level field name that happens to contain a period (.).

Use the $replaceWith pipeline stage with $setField and a nested $unsetField operation to remove the "euro" field:

db.inventory.aggregate( [   { $replaceWith: {        $setField: {           field: "price",           input: "$$ROOT",           value: {              $unsetField: {                 field: "euro",                 input: { $getField: "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 } }]

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