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$facet

The `$facet` stage allows for multiple parallel aggregations to be executed within a single pipeline stage. It's useful for performing multiple analyses on the same dataset in a single query.

Syntax

{
  $facet: {
    "outputField1": [{ "stage1": {} }, { "stage2": {} }],
    "outputField2": [{ "stage1": {} }, { "stage2": {} }]
  }
}

Parameters

outputFieldstringrequired

The name of the output field for each parallel pipeline.

stageobjectrequired

The aggregation stages to be executed in each pipeline.

Examples

Sample Data

{
  "_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
  "name": "First Up Consultants | Beverage Shop",
  "sales": {
    "salesByCategory": [
      { "categoryName": "Wine Accessories", "totalSales": 34440 }
    ]
  },
  "promotionEvents": [
    {
      "discounts": [
        { "categoryName": "Whiskey", "discountPercentage": 7 }
      ]
    }
  ]
}

Faceted search on sales and promotions

Perform simultaneous analyses on sales and promotions for specified product categories.

This query runs two parallel pipelines analyzing sales totals and average discounts.

Query:

db.stores.aggregate([
  {
    $facet: {
      salesAnalysis: [
        { $unwind: "$sales.salesByCategory" },
        { $match: { "sales.salesByCategory.categoryName": { $in: ["Laptops", "Smartphones"] } } },
        { $group: { _id: "$sales.salesByCategory.categoryName", totalSales: { $sum: "$sales.salesByCategory.totalSales" } } }
      ],
      promotionAnalysis: [
        { $unwind: "$promotionEvents" },
        { $unwind: "$promotionEvents.discounts" },
        { $match: { "promotionEvents.discounts.categoryName": { $in: ["Laptops", "Smartphones"] } } },
        { $group: { _id: "$promotionEvents.discounts.categoryName", avgDiscount: { $avg: "$promotionEvents.discounts.discountPercentage" } } }
      ]
    }
  }
])

Output:

[
  {
    "salesAnalysis": [
      { "_id": "Smartphones", "totalSales": 440815 },
      { "_id": "Laptops", "totalSales": 679453 }
    ],
    "promotionAnalysis": [
      { "_id": "Smartphones", "avgDiscount": 14.32 },
      { "_id": "Laptops", "avgDiscount": 14.78 }
    ]
  }
]

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