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$facet

The $facet stage allows you to run multiple aggregation pipelines in parallel on the same input documents. Each pipeline outputs to a separate field in the result.

Ideal for generating dashboards, side-by-side summaries, or multi-metric analysis.

📌 Syntax

{
  "$facet": {
    "facetName1": [ <pipeline1> ],
    "facetName2": [ <pipeline2> ],
    ...
  }
}

Each value must be a valid aggregation pipeline (array of stages).

📦 Ecommerce Example – Dashboard: Top Categories, Brands, and Price Segments

{
  "$facet": {
    "topCategories": [
      { "$group": { "_id": "$category", "totalSales": { "$sum": "$price" } } },
      { "$sort": { "totalSales": -1 } },
      { "$limit": 3 }
    ],
    "topBrands": [
      { "$group": { "_id": "$brand", "count": { "$sum": 1 } } },
      { "$sort": { "count": -1 } },
      { "$limit": 3 }
    ],
    "priceSegments": [
      {
        "$bucket": {
          "groupBy": "$price",
          "boundaries": [0, 100, 500, 1000],
          "default": "Other",
          "output": { "count": { "$sum": 1 } }
        }
      }
    ]
  }
}

Input Documents

[
  { "product": "Laptop", "price": 900, "category": "Electronics", "brand": "BrandA" },
  { "product": "Phone", "price": 600, "category": "Electronics", "brand": "BrandB" },
  { "product": "Shoes", "price": 75, "category": "Fashion", "brand": "BrandA" },
  { "product": "Book", "price": 20, "category": "Books", "brand": "BrandC" }
]

Output Document

{
  "topCategories": [
    { "_id": "Electronics", "totalSales": 1500 },
    { "_id": "Fashion", "totalSales": 75 },
    { "_id": "Books", "totalSales": 20 }
  ],
  "topBrands": [
    { "_id": "BrandA", "count": 2 },
    { "_id": "BrandB", "count": 1 },
    { "_id": "BrandC", "count": 1 }
  ],
  "priceSegments": [
    { "_id": 0, "count": 2 },
    { "_id": 500, "count": 1 },
    { "_id": 1000, "count": 1 }
  ]
}

âž• Supported Accumulators

Any accumulator supported by $group, $bucket, or $bucketAuto is usable inside $facet pipelines:

Accumulator Description
$sum Count or total values
$avg Mean price or rating
$min / $max Price extremes
$push, $addToSet Collect values

🔧 Operators in Expressions

Operators are used inside stages nested in each facet pipeline:

Operator Example
$multiply { "$multiply": ["$price", "$qty"] }
$cond { "$cond": { "if": ..., "then": ..., "else": ... } }
$map Transform arrays
$reduce Total prices

✅ Best Practices

  • Use $facet to aggregate once and branch for multiple metrics.
  • Combine $facet with $group, $bucket, $sort, $project, etc.
  • Avoid overly deep pipelines in each facet to keep it efficient.