$first

APPLIES TO: MongoDB vCore

The $first operator returns the first value in a group according to the group's sorting order. If no sorting order is specified, the order is undefined. It is useful when you want to retrieve the first document or value from each group.

Syntax

The syntax for the $first operator is as follows:

{ $first: <expression> }

Parameters

Description
<expression> The expression that specifies the value to return from the first document in each group.

Example

Let's understand the usage with sample json from stores dataset.

{
  "_id": "2cf3f885-9962-4b67-a172-aa9039e9ae2f",
  "name": "First Up Consultants | Bed and Bath Center - South Amir",
  "location": {
    "lat": 60.7954,
    "lon": -142.0012
  },
  "staff": {
    "totalStaff": {
      "fullTime": 18,
      "partTime": 17
    }
  },
  "sales": {
    "totalSales": 37701,
    "salesByCategory": [
      {
        "categoryName": "Mattress Toppers",
        "totalSales": 37701
      }
    ]
  },
  "promotionEvents": [
    {
      "eventName": "Price Drop Palooza",
      "promotionalDates": {
        "startDate": {
          "Year": 2024,
          "Month": 9,
          "Day": 21
        },
        "endDate": {
          "Year": 2024,
          "Month": 9,
          "Day": 30
        }
      }
    }
  ]
}

Example 1: Get first store by store name alphabetically

Get the first store when sorted alphabetically by store name.

db.stores.aggregate([
  { $sort: { "name": 1 } },
  {
    $group: {
      _id: null,
      firstStore: {
        $first: {
          storeId: "$_id",
          storeName: "$name",
          totalSales: "$sales.totalSales"
        }
      }
    }
  }
])

This will produce the following output:

[
  {
    _id: null,
    firstStore: {
      storeId: 'c53d4c75-e551-43a6-8752-d85d2a094be4',
      storeName: 'Adatum Corporation | Appliance Bargains - Bufordside',
      totalSales: 82598
    }
  }
]

Example 2: Get first category by sales amount per store

Get the first category (alphabetically) for each store along with store details.

db.stores.aggregate([
  { $unwind: "$sales.salesByCategory" },
  { $sort: { "_id": 1, "sales.salesByCategory.categoryName": 1 } },
  {
    $group: {
      _id: "$_id",
      storeName: { $first: "$name" },
      totalSales: { $first: "$sales.totalSales" },
      firstCategory: {
        $first: {
          categoryName: "$sales.salesByCategory.categoryName",
          categoryTotalSales: "$sales.salesByCategory.totalSales"
        }
      }
    }
  }
])

This will produce output showing the first category alphabetically for each store:

[
  {
    _id: '64ec6589-068a-44a6-be5b-9d37bb0a90f1',
    storeName: 'First Up Consultants | Computer Gallery - West Cathrine',
    totalSales: 186829,
    firstCategory: { categoryName: 'Cases', categoryTotalSales: 36386 }
  },
  {
    _id: '14343900-2a5c-44bf-a52b-9efe63579866',
    storeName: 'Northwind Traders | Home Improvement Closet - West Evanside',
    totalSales: 35371,
    firstCategory: { categoryName: 'Doors', categoryTotalSales: 21108 }
  },
.
.
.
]

Example 3: Get first promotion event per store

Get the first promotion event for each store based on start date.

db.stores.aggregate([
  { $unwind: "$promotionEvents" },
  { 
    $sort: { 
      "_id": 1, 
      "promotionEvents.promotionalDates.startDate.Year": 1,
      "promotionEvents.promotionalDates.startDate.Month": 1,
      "promotionEvents.promotionalDates.startDate.Day": 1
    }
  },
  {
    $group: {
      _id: "$_id",
      storeName: { $first: "$name" },
      firstPromotionEvent: {
        $first: {
          eventName: "$promotionEvents.eventName",
          startYear: "$promotionEvents.promotionalDates.startDate.Year",
          startMonth: "$promotionEvents.promotionalDates.startDate.Month"
        }
      }
    }
  }
])

This will return the earliest promotion event for each store based on the start date.

[
  {
    _id: '64ec6589-068a-44a6-be5b-9d37bb0a90f1',
    storeName: 'First Up Consultants | Computer Gallery - West Cathrine',
    firstPromotionEvent: {
      eventName: 'Crazy Markdown Madness',
      startYear: 2024,
      startMonth: 6
    }
  },
  {
    _id: 'a58d0356-493b-44e6-afab-260aa3296930',
    storeName: 'Fabrikam, Inc. | Outdoor Furniture Nook - West Lexie',
    firstPromotionEvent: { eventName: 'Price Drop Palooza', startYear: 2023, startMonth: 9 }
  },
.
.
.
]

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