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Add reference Markdown: operators/array-update #42

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226 changes: 226 additions & 0 deletions reference/operators/array-update/$.md
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---
title: $
description: The $ positional operator identifies an element in an array to update without explicitly specifying the position of the element in the array.
type: operators
category: array-update
---

# $

The `$` positional operator identifies an element in an array to update without explicitly specifying the position of the element in the array. The `$` operator acts as a placeholder for the first element that matches the query condition, and the array field must appear as part of the query document.

## Syntax

```javascript
db.collection.updateOne(
{ <array>: <value> },
{ <update operator>: { "<array>.$": <value> } }
)
```

## Parameters

| Parameter | Description |
| --- | --- |
| **`array`** | The array field that contains the element to update. Must be part of the query condition. |
| **`value`** | The value used to match the array element in the query condition. |
| **`update operator`** | The update operator to apply (for example, `$set`, `$inc`, `$unset`). |

## Examples

Consider this sample document from the stores collection.

```json
{
"_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
"name": "First Up Consultants | Beverage Shop - Satterfieldmouth",
"location": {
"lat": -89.2384,
"lon": -46.4012
},
"staff": {
"totalStaff": {
"fullTime": 8,
"partTime": 20
}
},
"sales": {
"totalSales": 75670,
"salesByCategory": [
{
"categoryName": "Wine Accessories",
"totalSales": 34440
},
{
"categoryName": "Bitters",
"totalSales": 39496
},
{
"categoryName": "Rum",
"totalSales": 1734
}
]
},
"promotionEvents": [
{
"eventName": "Unbeatable Bargain Bash",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 6,
"Day": 23
},
"endDate": {
"Year": 2024,
"Month": 7,
"Day": 2
}
},
"discounts": [
{
"categoryName": "Whiskey",
"discountPercentage": 7
},
{
"categoryName": "Bitters",
"discountPercentage": 15
},
{
"categoryName": "Brandy",
"discountPercentage": 8
},
{
"categoryName": "Sports Drinks",
"discountPercentage": 22
},
{
"categoryName": "Vodka",
"discountPercentage": 19
}
]
},
{
"eventName": "Steal of a Deal Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 9,
"Day": 21
},
"endDate": {
"Year": 2024,
"Month": 9,
"Day": 29
}
},
"discounts": [
{
"categoryName": "Organic Wine",
"discountPercentage": 19
},
{
"categoryName": "White Wine",
"discountPercentage": 20
},
{
"categoryName": "Sparkling Wine",
"discountPercentage": 19
},
{
"categoryName": "Whiskey",
"discountPercentage": 17
},
{
"categoryName": "Vodka",
"discountPercentage": 23
}
]
}
]
}
```

### Example 1: Project the first element of an array, matching the condition

This query returns the first element within the `salesByCategory` array, for `DJ` equipment with `totalSales` greater than 35000.

```javascript
db.stores.find({
"sales.salesByCategory": {
$elemMatch: {
categoryName: {
$regex: "^DJ"
}
}
},
"sales.salesByCategory.totalSales": {
$gt: 35000
}
}, {
"sales.salesByCategory.$": 1
}).limit(2)
```

The first two results returned by this query are:

```json
[
{
"_id": "d3c9df51-41bd-4b4e-a26b-b038d9cf8b45",
"sales": {
"salesByCategory": [
{
"categoryName": "DJ Speakers",
"totalSales": 36972
}
]
}
},
{
"_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
"sales": {
"salesByCategory": [
{
"categoryName": "DJ Headphones",
"totalSales": 35911
}
]
}
}
]
```

### Example 2: Update discount percentage for a specific category

This query updates the discount percentage for "Desks" category in the first matching promotion event.

```javascript
db.stores.updateOne(
{
_id: "905d1939-e03a-413e-a9c4-221f74055aac",
"promotionEvents.discounts.categoryName": "Desks"
},
{
$set: { "promotionEvents.$.discounts.$[elem].discountPercentage": 25 }
},
{
arrayFilters: [{ "elem.categoryName": "Desks" }]
}
)
```

### Example 3: Update sales category total

This query updates the total sales for a specific category using the `$ (positional operator)`.

```javascript
db.stores.updateOne(
{
_id: "905d1939-e03a-413e-a9c4-221f74055aac",
"sales.salesByCategory.categoryName": "Desk Lamps"
},
{
$inc: { "sales.salesByCategory.$.totalSales": 1000 }
}
)
```
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