$unset

Definition

$unset

The $unset operator deletes a particular field. Consider the following syntax:

{ $unset: { <field1>: "", ... } }

The specified value in the $unset expression (i.e. "") does not impact the operation.

To specify a <field> in an embedded document or in an array, use dot notation.

Behavior

Starting in MongoDB 5.0, update operators process document fields with string-based names in lexicographic order. Fields with numeric names are processed in numeric order. See Update Operators Behavior for details.

If the field does not exist, then $unset does nothing (i.e. no operation).

When used with $ to match an array element, $unset replaces the matching element with null rather than removing the matching element from the array. This behavior keeps consistent the array size and element positions.

Starting in MongoDB 5.0, mongod no longer raises an error when you use an update operator like $unset with an empty operand expression ( { } ). An empty update results in no changes and no oplog entry is created (meaning that the operation is a no-op).

Example

Create the products collection:

db.products.insertMany( [
   { "item": "chisel", "sku": "C001", "quantity": 4, "instock": true },
   { "item": "hammer", "sku": "unknown", "quantity": 3, "instock": true },
   { "item": "nails", "sku": "unknown", "quantity": 100, "instock": true }
] )

Update the first document in the products collection where the value of sku is unknown:

db.products.updateOne(
   { sku: "unknown" },
   { $unset: { quantity: "", instock: "" } }
)

updateOne() uses the $unset operator to:

• remove the quantity field

• remove the instock field

{
  item: 'chisel',
  sku: 'C001',
  quantity: 4,
  instock: true
},
{
  item: 'hammer',
  sku: 'unknown'
},
{
  item: 'nails',
  sku: 'unknown',
  quantity: 100,
  instock: true
}