$inc

Definition

$inc

The $inc operator increments a field by a specified value and has the following form:

{ $inc: { <field1>: <amount1>, <field2>: <amount2>, ... } }

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.

The $inc operator accepts positive and negative values.

If the field does not exist, $inc creates the field and sets the field to the specified value.

Use of the $inc operator on a field with a null value will generate an error.

$inc is an atomic operation within a single document.

Starting in MongoDB 5.0, mongod no longer raises an error when you use an update operator like $inc 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.insertOne(
   {
     _id: 1,
     sku: "abc123",
     quantity: 10,
     metrics: { orders: 2, ratings: 3.5 }
   }
)

The following updateOne() operation uses the $inc operator to:

• increase the "metrics.orders" field by 1

• increase the quantity field by -2 (which decreases quantity)

db.products.updateOne(
   { sku: "abc123" },
   { $inc: { quantity: -2, "metrics.orders": 1 } }
)

The updated document would resemble:

{
  _id: 1,
  sku: 'abc123',
  quantity: 8,
  metrics: { orders: 3, ratings: 3.5 }
}