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$bucket (aggregation)

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Definition

$bucket

New in version 3.4.

Categorizes incoming documents into groups, called buckets, based on a specified expression and bucket boundaries.

Each bucket is represented as a document in the output. The document for each bucket contains an _id field, whose value specifies the inclusive lower bound of the bucket and a count field that contains the number of documents in the bucket. The count field is included by default when the output is not specified.

$bucket only produces output documents for buckets that contain at least one input document.

{
  $bucket: {
      groupBy: <expression>,
      boundaries: [ <lowerbound1>, <lowerbound2>, ... ],
      default: <literal>,
      output: {
         <output1>: { <$accumulator expression> },
         ...
         <outputN>: { <$accumulator expression> }
      }
   }
}
Field Type Description
groupBy expression

An expression to group documents by. To specify a field path, prefix the field name with a dollar sign $ and enclose it in quotes.

Unless $bucket includes a default specification, each input document must resolve the groupBy field path or expression to a value that falls within one of the ranges specified by the boundaries.

boundaries array

An array of values based on the groupBy expression that specify the boundaries for each bucket. Each adjacent pair of values acts as the inclusive lower boundary and the exclusive upper boundary for the bucket. You must specify at least two boundaries.

The specified values must be in ascending order and all of the same type. The exception is if the values are of mixed numeric types, such as:

[ 10, NumberLong(20), NumberInt(30) ]

If the values are documents, they must be wrapped in the $literal operator.

Example

An array of [ 0, 5, 10 ] creates two buckets:

  • [0, 5) with inclusive lower bound 0 and exclusive upper bound 5.
  • [5, 10) with inclusive lower bound 5 and exclusive upper bound 10.
default literal

Optional. A literal that specifies the _id of an additional bucket that contains all documents whose groupBy expression result does not fall into a bucket specified by boundaries.

If unspecified, each input document must resolve the groupBy expression to a value within one of the bucket ranges specified by boundaries or the operation throws an error.

The default value cannot overlap with a bucket; i.e. must be less than the lowest boundaries value or greater than the highest boundaries value.

The default value can be of a different type than the entries in boundaries.

output document

Optional. A document that specifies the fields to include in the output documents in addition to the _id field. To specify the field to include, you must use accumulator expressions.

<outputfield1>: { <accumulator>: <expression1> },
...
<outputfieldN>: { <accumulator>: <expressionN> }

The default count field is not included in the output document when output is specified. Explicitly specify the count expression as part of the output document to include it:

output: {
  <outputField1>: { <accumulator>: <expression1> },
  ...
  "count": { $sum: 1 }
}

Behavior

$bucket requires at least one of the following conditions to be met or the operation throws an error:

  • Each input document resolves the groupBy expression to a value within one of the bucket ranges specified by boundaries, or
  • A default value is specified to bucket documents whose groupBy values are outside of the boundaries or of a different BSON type than the values in boundaries.

If the groupBy expression resolves to an array or a document, $bucket arranges the input documents into buckets using the comparison logic from $sort.

Example

Consider a collection artwork with the following documents:

{ "_id" : 1, "title" : "The Pillars of Society", "artist" : "Grosz", "year" : 1926,
    "price" : NumberDecimal("199.99") }
{ "_id" : 2, "title" : "Melancholy III", "artist" : "Munch", "year" : 1902,
    "price" : NumberDecimal("280.00") }
{ "_id" : 3, "title" : "Dancer", "artist" : "Miro", "year" : 1925,
    "price" : NumberDecimal("76.04") }
{ "_id" : 4, "title" : "The Great Wave off Kanagawa", "artist" : "Hokusai",
    "price" : NumberDecimal("167.30") }
{ "_id" : 5, "title" : "The Persistence of Memory", "artist" : "Dali", "year" : 1931,
    "price" : NumberDecimal("483.00") }
{ "_id" : 6, "title" : "Composition VII", "artist" : "Kandinsky", "year" : 1913,
    "price" : NumberDecimal("385.00") }
{ "_id" : 7, "title" : "The Scream", "artist" : "Munch", "year" : 1893
    /* No price*/ }
{ "_id" : 8, "title" : "Blue Flower", "artist" : "O'Keefe", "year" : 1918,
    "price" : NumberDecimal("118.42") }

The following operation uses the $bucket aggregation stage to arrange the artwork collection into buckets according to price:

db.artwork.aggregate( [
  {
    $bucket: {
      groupBy: "$price",
      boundaries: [ 0, 200, 400 ],
      default: "Other",
      output: {
        "count": { $sum: 1 },
        "titles" : { $push: "$title" }
      }
    }
  }
] )

The buckets have the following boundaries:

  • [0, 200) with inclusive lowerbound 0 and exclusive upper bound 200.
  • [200, 400) with inclusive lowerbound 200 and exclusive upper bound 400.
  • “Other” is the default bucket for documents without prices or prices outside the ranges above.

The operation returns the following documents:

{
  "_id" : 0,
  "count" : 4,
  "titles" : [
    "The Pillars of Society",
    "Dancer",
    "The Great Wave off Kanagawa",
    "Blue Flower"
  ]
}
{
  "_id" : 200,
  "count" : 2,
  "titles" : [
    "Melancholy III",
    "Composition VII"
  ]
}
{
  "_id" : "Other",
  "count" : 2,
  "titles" : [
    "The Persistence of Memory",
    "The Scream"
  ]
}

Using $bucket with $facet

The $bucket stage can be used within the $facet stage to process multiple aggregation pipelines on artwork in a single aggregation stage.

The following operation groups the documents from artwork into buckets based on price and year:

db.artwork.aggregate( [
  {
    $facet: {
      "price": [
        {
          $bucket: {
              groupBy: "$price",
              boundaries: [ 0, 200, 400 ],
              default: "Other",
              output: {
                "count": { $sum: 1 },
                "artwork" : { $push: { "title": "$title", "price": "$price" } }
              }
          }
        }
      ],
      "year": [
        {
          $bucket: {
            groupBy: "$year",
            boundaries: [ 1890, 1910, 1920, 1940 ],
            default: "Unknown",
            output: {
              "count": { $sum: 1 },
              "artwork": { $push: { "title": "$title", "year": "$year" } }
            }
          }
        }
      ]
    }
  }
] )

The first facet groups the input documents by price. The buckets have the following boundaries:

  • [0, 200) with inclusive lowerbound 0 and exclusive upper bound 200.
  • [200, 400) with inclusive lowerbound 200 and exclusive upper bound 400.
  • “Other”, the``default`` bucket containing documents without prices or prices outside the ranges above.

The second facet groups the input documents by year. The buckets have the following boundaries:

  • [1890, 1910) with inclusive lowerbound 1890 and exclusive upper bound 1910.
  • [1910, 1920) with inclusive lowerbound 1910 and exclusive upper bound 1920.
  • [1920, 1940) with inclusive lowerbound 1910 and exclusive upper bound 1940.
  • “Unknown”, the``default`` bucket containing documents without years or years outside the ranges above.

The operation returns the following document:

{
  "year" : [
    {
      "_id" : 1890,
      "count" : 2,
      "artwork" : [
        {
          "title" : "Melancholy III",
          "year" : 1902
        },
        {
          "title" : "The Scream",
          "year" : 1893
        }
      ]
    },
    {
      "_id" : 1910,
      "count" : 2,
      "artwork" : [
        {
          "title" : "Composition VII",
          "year" : 1913
        },
        {
          "title" : "Blue Flower",
          "year" : 1918
        }
      ]
    },
    {
      "_id" : 1920,
      "count" : 3,
      "artwork" : [
        {
          "title" : "The Pillars of Society",
          "year" : 1926
        },
        {
          "title" : "Dancer",
          "year" : 1925
        },
        {
          "title" : "The Persistence of Memory",
          "year" : 1931
        }
      ]
    },
    {
      // Includes the document without a year, e.g., _id: 4
      "_id" : "Other",
      "count" : 1,
      "artwork" : [
        {
          "title" : "The Great Wave off Kanagawa"
        }
      ]
    }
  ],
      "price" : [
    {
      "_id" : 0,
      "count" : 4,
      "artwork" : [
        {
          "title" : "The Pillars of Society",
          "price" : NumberDecimal("199.99")
        },
        {
          "title" : "Dancer",
          "price" : NumberDecimal("76.04")
        },
        {
          "title" : "The Great Wave off Kanagawa",
          "price" : NumberDecimal("167.30")
        },
        {
          "title" : "Blue Flower",
          "price" : NumberDecimal("118.42")
        }
      ]
    },
    {
      "_id" : 200,
      "count" : 2,
      "artwork" : [
        {
          "title" : "Melancholy III",
          "price" : NumberDecimal("280.00")
        },
        {
          "title" : "Composition VII",
          "price" : NumberDecimal("385.00")
        }
      ]
    },
    {
      // Includes the document without a price, e.g., _id: 7
      "_id" : "Other",
      "count" : 2,
      "artwork" : [
        {
          "title" : "The Persistence of Memory",
          "price" : NumberDecimal("483.00")
        },
        {
          "title" : "The Scream"
        }
      ]
    }
  ]
}

See also $bucketAuto