How to use MongoDB $unset operator to remove a field from a document

Summary: in this tutorial, you’ll learn how to use the MongoDB $unset operator to remove one or more fields from a document.

Introduction to the MongoDB $unset operator

Sometimes, you may want to remove one or more fields from a document. In order to do it, you can use the $unset operator.

The $unset is a field update operator that completely removes a particular field from a document.

The $unset operator has the following syntax:

{ $unset: {<field>: "", ... }}
Code language: PHP (php)

In this syntax, you specify the field that you want to remove and its value. The field value isn’t important and doesn’t impact the operation. You can specify any value, the $unset will remove the field completely.

If the <field> doesn’t exist in the document, then $unset operator will do nothing. It also won’t issue any warnings or errors.

To specify a field in an embedded document, you use the dot notation like this:

{ $unset: { "<embedded_doc>.<field>: "", ... }}
Code language: HTML, XML (xml)

Note that the $unset operator doesn’t remove array elements. Instead, it sets the array elements to null.

{ $unset: {"<array>.<index>": "", ...}
Code language: PHP (php)

This behavior keeps the array size and element positions consistent.

MongoDB $unset operator examples

We’ll use the following products collection:

db.products.insertMany([ { "_id" : 1, "name" : "xPhone", "price" : 799, "releaseDate": ISODate("2011-05-14"), "spec" : { "ram" : 4, "screen" : 6.5, "cpu" : 2.66 },"color":["white","black"],"storage":[64,128,256]}, { "_id" : 2, "name" : "xTablet", "price" : 899, "releaseDate": ISODate("2011-09-01") , "spec" : { "ram" : 16, "screen" : 9.5, "cpu" : 3.66 },"color":["white","black","purple"],"storage":[128,256,512]}, { "_id" : 3, "name" : "SmartTablet", "price" : 899, "releaseDate": ISODate("2015-01-14"), "spec" : { "ram" : 12, "screen" : 9.7, "cpu" : 3.66 },"color":["blue"],"storage":[16,64,128]}, { "_id" : 4, "name" : "SmartPad", "price" : 699, "releaseDate": ISODate("2020-05-14"),"spec" : { "ram" : 8, "screen" : 9.7, "cpu" : 1.66 },"color":["white","orange","gold","gray"],"storage":[128,256,1024]}, { "_id" : 5, "name" : "SmartPhone", "price" : 599,"releaseDate": ISODate("2022-09-14"), "spec" : { "ram" : 4, "screen" : 5.7, "cpu" : 1.66 },"color":["white","orange","gold","gray"],"storage":[128,256]} ])
Code language: JavaScript (javascript)

1) Using the MongoDB $unset operator to remove a field from a document

The following example uses the $unset operator to remove price field from the document _id 1 in the products collection:

db.products.updateOne({ _id: 1 }, { $unset: { price: "" } })
Code language: PHP (php)

Output:

{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }
Code language: JSON / JSON with Comments (json)

The modifiedCount indicated that one document has been modified. This query returns all documents from the products collection to verify the update:

db.products.find({}, { name: 1, price: 1 })
Code language: CSS (css)

Output:

{ "_id" : 1, "name" : "xPhone" } { "_id" : 2, "name" : "xTablet", "price" : 899 } { "_id" : 3, "name" : "SmartTablet", "price" : 899 } { "_id" : 4, "name" : "SmartPad", "price" : 699 } { "_id" : 5, "name" : "SmartPhone", "price" : 599 }
Code language: JSON / JSON with Comments (json)

As you can see clearly from the output, the $unset operator has completely removed the price field from the document with _id 1.

2) Using the MongoDB $unset operator to remove a field in an embedded document

The following statement uses the $unset operator to remove the ram field from the spec embedded documents of all documents in the products collection:

db.products.updateMany({}, { $unset: { "spec.ram": "" } })
Code language: PHP (php)

Output:

{ "acknowledged" : true, "matchedCount" : 5, "modifiedCount" : 5 }
Code language: JSON / JSON with Comments (json)

The following query returns all documents from the products collection:

db.products.find({}, { spec: 1 })
Code language: CSS (css)

Output:

{ "_id" : 1, "spec" : { "screen" : 6.5, "cpu" : 2.66 } } { "_id" : 2, "spec" : { "screen" : 9.5, "cpu" : 3.66 } } { "_id" : 3, "spec" : { "screen" : 9.7, "cpu" : 3.66 } } { "_id" : 4, "spec" : { "screen" : 9.7, "cpu" : 1.66 } } { "_id" : 5, "spec" : { "screen" : 5.7, "cpu" : 1.66 } }
Code language: JSON / JSON with Comments (json)

As you can see, the ram field have been removed from spec embedded document in all documents.

3) Using the MongoDB $unset operator to set array elements to null

The following example uses the $unset operator to set the first elements of the storage arrays to null:

db.products.updateMany({}, { $unset: { "storage.0": "" } })
Code language: PHP (php)

Output:

{ "acknowledged" : true, "matchedCount" : 5, "modifiedCount" : 5 }
Code language: JSON / JSON with Comments (json)

The following query selects the storage array from all documents in the products collection:

db.products.updateMany({}, { $unset: { "storage.0": "" } })
Code language: PHP (php)

Output:

{ "_id" : 1, "storage" : [ null, 128, 256 ] } { "_id" : 2, "storage" : [ null, 256, 512 ] } { "_id" : 3, "storage" : [ null, 64, 128 ] } { "_id" : 4, "storage" : [ null, 256, 1024 ] } { "_id" : 5, "storage" : [ null, 256 ] }
Code language: JSON / JSON with Comments (json)

In this example, the $unset operator sets the first elements of the storage arrays to null instead of removing them completely.

4) Using the MongoDB $unset operator to remove multiple fields from a document

The following statement uses the $unset operator to remove the releaseDate and spec fields from all the documents in the products collection:

db.products.updateMany({}, { $unset: { releaseDate: "", spec: "" } })
Code language: PHP (php)

Output:

{ "acknowledged" : true, "matchedCount" : 5, "modifiedCount" : 5 }
Code language: JSON / JSON with Comments (json)

The following query verifies the update:

db.products.find({}, { name: 1, storage: 1, releaseDate: 1, spec: 1 })
Code language: CSS (css)

Output:

{ "_id" : 1, "name" : "xPhone", "storage" : [ null, 128, 256 ] } { "_id" : 2, "name" : "xTablet", "storage" : [ null, 256, 512 ] } { "_id" : 3, "name" : "SmartTablet", "storage" : [ null, 64, 128 ] } { "_id" : 4, "name" : "SmartPad", "storage" : [ null, 256, 1024 ] } { "_id" : 5, "name" : "SmartPhone", "storage" : [ null, 256 ] }
Code language: JSON / JSON with Comments (json)

As shown clearly from the output, the releaseDate and spec fields now are gone.

Summary

  • Use the $unset operator to completely remove a field from a document.
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