$map
The $map operator allows applying an expression to each element in an array.
$map
The $map operator is used to apply an expression to each element in an array and return an array with the applied results. This operator is useful for transforming arrays within documents, such as modifying each element or extracting specific fields.
Parameters
| Parameter | Description |
|---|---|
input | The array processed by the expression. |
as | The variable name for each element in the array. |
in | The expression to apply to each element. |
Examples
Consider this sample document from the stores collection.
{
"_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
"name": "Lakeshore Retail",
"location": {
"lat": -51.3041,
"lon": -166.0838
},
"staff": {
"totalStaff": {
"fullTime": 5,
"partTime": 20
}
},
"sales": {
"totalSales": 266491,
"salesByCategory": [
{
"categoryName": "Towel Racks",
"totalSales": 13237
},
{
"categoryName": "Washcloths",
"totalSales": 44315
},
{
"categoryName": "Face Towels",
"totalSales": 42095
},
{
"categoryName": "Toothbrush Holders",
"totalSales": 47912
},
{
"categoryName": "Hybrid Mattresses",
"totalSales": 48660
},
{
"categoryName": "Napkins",
"totalSales": 31439
},
{
"categoryName": "Pillow Cases",
"totalSales": 38833
}
]
},
"tag": [
"#ShopLocal",
"#FashionStore",
"#SeasonalSale",
"#FreeShipping",
"#MembershipDeals"
]
}Example 1: Extracting category names
This query filters the stores collection for _id, then projects a new field categoryNames where each element in the salesByCategory array has its totalSales increased by 500 using the $map operator.
db.stores.aggregate([{
$match: {
_id: "988d2dd1-2faa-4072-b420-b91b95cbfd60"
}
},
{
$project: {
categoryNames: {
$map: {
input: "$sales.salesByCategory.totalSales",
as: "category",
in: {
$add: ["$$category", 500]
}
}
}
}
}
])This query returns the following result.
[
{
"_id": "988d2dd1-2faa-4072-b420-b91b95cbfd60",
"categoryNames": [
13737,
44815,
42595,
48412,
49160,
31939,
39333
]
}
]