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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

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.

Example

Consider a collection products with the following document:

{
  _id: 1,
  sku: "abc123",
  quantity: 10,
  metrics: {
    orders: 2,
    ratings: 3.5
  }
}

The following update() operation uses the $inc operator to decrease the quantity field by 2 (i.e. increase by -2) and increase the "metrics.orders" field by 1:

db.products.update(
   { 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
   }
}