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Add reference Markdown: operators/array-expression #40

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179 changes: 179 additions & 0 deletions reference/operators/array-expression/$arrayelemat.md
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---
title: $arrayElemAt
description: The $arrayElemAt returns the element at the specified array index.
type: operators
category: array-expression
---

# $arrayElemAt

The `$arrayElemAt` operator is used to return the element at the specified array index. This operator is helpful when you need to extract a specific element from an array within your documents.

## Syntax

```javascript
{
$arrayElemAt: ["<array>", <idx>]
}
```

## Parameters

| Parameter | Description |
| --- | --- |
| **`<array>`**| The array reference from which the element is retrieved.|
| **`<idx>`**| The index of the element to return. The index is zero-based. A negative index counts from the end of the array.|

## Examples

Consider this sample document from the stores collection.

```json
{
"_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
"name": "Lakeshore Retail | DJ Equipment Stop - Port Cecile",
"location": {
"lat": 60.1441,
"lon": -141.5012
},
"staff": {
"totalStaff": {
"fullTime": 2,
"partTime": 0
}
},
"sales": {
"salesByCategory": [
{
"categoryName": "DJ Cables",
"totalSales": 1000
},
{
"categoryName": "DJ Headphones",
"totalSales": 35921
}
],
"fullSales": 3700
},
"promotionEvents": [
{
"eventName": "Cyber Monday Event",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 8,
"Day": 1
},
"endDate": {
"Year": 2024,
"Month": 8,
"Day": 7
}
},
"discounts": [
{
"categoryName": "DJ Speakers",
"discountPercentage": 25
}
]
},
{
"eventName": "Black Friday Event",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 8,
"Day": 1
},
"endDate": {
"Year": 2024,
"Month": 8,
"Day": 7
}
},
"discounts": [
{
"categoryName": "DJ Speakers",
"discountPercentage": 25
}
]
},
{
"eventName": "Mega Discount Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 5,
"Day": 11
},
"endDate": {
"Year": 2024,
"Month": 5,
"Day": 18
}
},
"discounts": [
{
"categoryName": "DJ Lights",
"discountPercentage": 20
}
]
}
],
"tag": [
"#ShopLocal",
"#NewArrival",
"#FashionStore",
"#SeasonalSale",
"#FreeShipping",
"#MembershipDeals"
]
}

```

### Example 1: Return the first element from an array field

This query retrieves the first event details from `promotionEvents` array for the searched store.

```javascript
db.stores.aggregate([
{ $match: { name: "Lakeshore Retail | DJ Equipment Stop - Port Cecile" } },
{
$project: {
firstPromotionEvent: { $arrayElemAt: ["$promotionEvents", 0] }
}
}
])
```

This query returns the following result.

```json
[
{
"_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
"firstPromotionEvent": {
"eventName": "Cyber Monday Event",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 8,
"Day": 1
},
"endDate": {
"Year": 2024,
"Month": 8,
"Day": 7
}
},
"discounts": [
{
"categoryName": "DJ Speakers",
"discountPercentage": 25
}
]
}
}
]
```
152 changes: 152 additions & 0 deletions reference/operators/array-expression/$arraytoobject.md
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---
title: $arrayToObject
description: The $arrayToObject allows converting an array into a single document.
type: operators
category: array-expression
---

# $arrayToObject

The `$arrayToObject` operator is used to convert an array into a single document. This operator is useful when you need to transform arrays of key-value pairs into a more structured document format.

## Syntax

```javascript
{
$arrayToObject: "<array>"
}
```

## Parameters

| Parameter | Description |
| --- | --- |
| **`<array>`**| The array to be converted into a document. Each element in the array must be either: a) A two-element array where the first element is the field name and the second element is the field value. b) A document with exactly two fields, "k" and "v", where "k" is the field name and "v" is the field value.|

## Examples

Consider this sample document from the stores collection.

```json
{
"_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
"name": "Lakeshore Retail | DJ Equipment Stop - Port Cecile",
"location": {
"lat": 60.1441,
"lon": -141.5012
},
"staff": {
"totalStaff": {
"fullTime": 2,
"partTime": 0
}
},
"sales": {
"salesByCategory": [
{
"categoryName": "DJ Headphones",
"totalSales": 35921
},
{
"categoryName": "DJ Cables",
"totalSales": 1000
}
],
"fullSales": 3700
},
"promotionEvents": [
{
"eventName": "Bargain Blitz Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 3,
"Day": 11
},
"endDate": {
"Year": 2024,
"Month": 2,
"Day": 18
}
},
"discounts": [
{
"categoryName": "DJ Turntables",
"discountPercentage": 18
},
{
"categoryName": "DJ Mixers",
"discountPercentage": 15
}
]
},
{
"eventName": "Discount Delight Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 5,
"Day": 11
},
"endDate": {
"Year": 2024,
"Month": 5,
"Day": 18
}
}
}
],
"tag": [
"#ShopLocal",
"#FashionStore",
"#SeasonalSale",
"#FreeShipping",
"#MembershipDeals"
]
}
```

### Example 1: Convert the array to a key: value document

This query converts the `salesByCategory` array into an object where each `categoryName` is a key and `totalSales` is the corresponding value. This transformation simplifies access to sales data by category directly from an object structure.

```javascript
db.stores.aggregate([{
$match: {
_id: "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5"
}
},
{
$project: {
"sales.salesByCategory": {
$arrayToObject: {
$map: {
input: "$sales.salesByCategory",
as: "item",
in: {
k: "$$item.categoryName",
v: "$$item.totalSales"
}
}
}
}
}
}
])
```

The query returns the following result.

```json
[
{
"_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
"sales": {
"salesByCategory": {
"DJ Headphones": 35921,
"DJ Cables": 1000
}
}
}
]
```
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