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$bucket (aggregation)

$bucket

New in version 3.4.

Categorizes incoming documents into groups, called buckets, based on a specified expression and bucket boundaries and outputs a document per each bucket. Each output document contains an _id field whose value specifies the inclusive lower bound of the bucket. The output option specifies the fields included in each output document.

$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> }
}
}
}

The $bucket document contains the following fields:

Field
Type
Description
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.

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) ]

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.
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 must be less than the lowest boundaries value, or greater than or equal to the highest boundaries value.

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

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> }

If you do not specify an output document, the operation returns a count field containing the number of documents in each bucket.

If you specify an output document, only the fields specified in the document are returned; i.e. the count field is not returned unless it is explicitly included in the output document.

$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.

From the mongo shell, create a sample collection named artists with the following documents:

db.artists.insertMany([
{ "_id" : 1, "last_name" : "Bernard", "first_name" : "Emil", "year_born" : 1868, "year_died" : 1941, "nationality" : "France" },
{ "_id" : 2, "last_name" : "Rippl-Ronai", "first_name" : "Joszef", "year_born" : 1861, "year_died" : 1927, "nationality" : "Hungary" },
{ "_id" : 3, "last_name" : "Ostroumova", "first_name" : "Anna", "year_born" : 1871, "year_died" : 1955, "nationality" : "Russia" },
{ "_id" : 4, "last_name" : "Van Gogh", "first_name" : "Vincent", "year_born" : 1853, "year_died" : 1890, "nationality" : "Holland" },
{ "_id" : 5, "last_name" : "Maurer", "first_name" : "Alfred", "year_born" : 1868, "year_died" : 1932, "nationality" : "USA" },
{ "_id" : 6, "last_name" : "Munch", "first_name" : "Edvard", "year_born" : 1863, "year_died" : 1944, "nationality" : "Norway" },
{ "_id" : 7, "last_name" : "Redon", "first_name" : "Odilon", "year_born" : 1840, "year_died" : 1916, "nationality" : "France" },
{ "_id" : 8, "last_name" : "Diriks", "first_name" : "Edvard", "year_born" : 1855, "year_died" : 1930, "nationality" : "Norway" }
])

The following operation groups the documents into buckets according to the year_born field and filters based on the count of documents in the buckets:

db.artists.aggregate( [
// First Stage
{
$bucket: {
groupBy: "$year_born", // Field to group by
boundaries: [ 1840, 1850, 1860, 1870, 1880 ], // Boundaries for the buckets
default: "Other", // Bucket id for documents which do not fall into a bucket
output: { // Output for each bucket
"count": { $sum: 1 },
"artists" :
{
$push: {
"name": { $concat: [ "$first_name", " ", "$last_name"] },
"year_born": "$year_born"
}
}
}
}
},
// Second Stage
{
$match: { count: {$gt: 3} }
}
] )
First Stage

The $bucket stage groups the documents into buckets by the year_born field. The buckets have the following boundaries:

  • [1840, 1850) with inclusive lowerbound 1840 and exclusive upper bound 1850.
  • [1850, 1860) with inclusive lowerbound 1850 and exclusive upper bound 1860.
  • [1860, 1870) with inclusive lowerbound 1860 and exclusive upper bound 1870.
  • [1870, 1880) with inclusive lowerbound 1870 and exclusive upper bound 1880.
  • If a document did not contain the year_born field or its year_born field was outside the ranges above, it would be placed in the default bucket with the _id value "Other".

The stage includes the output document to determine the fields to return:

Field
Description
_id
Inclusive lower bound of the bucket.
count
Count of documents in the bucket.
artists

Array of documents containing information on each artist in the bucket. Each document contains the artist's

  • name, which is a concatenation (i.e. $concat) of the artist's first_name and last_name.
  • year_born

This stage passes the following documents to the next stage:

{ "_id" : 1840, "count" : 1, "artists" : [ { "name" : "Odilon Redon", "year_born" : 1840 } ] }
{ "_id" : 1850, "count" : 2, "artists" : [ { "name" : "Vincent Van Gogh", "year_born" : 1853 },
{ "name" : "Edvard Diriks", "year_born" : 1855 } ] }
{ "_id" : 1860, "count" : 4, "artists" : [ { "name" : "Emil Bernard", "year_born" : 1868 },
{ "name" : "Joszef Rippl-Ronai", "year_born" : 1861 },
{ "name" : "Alfred Maurer", "year_born" : 1868 },
{ "name" : "Edvard Munch", "year_born" : 1863 } ] }
{ "_id" : 1870, "count" : 1, "artists" : [ { "name" : "Anna Ostroumova", "year_born" : 1871 } ] }
Second Stage

The $match stage filters the output from the previous stage to only return buckets which contain more than 3 documents.

The operation returns the following document:

{ "_id" : 1860, "count" : 4, "artists" :
[
{ "name" : "Emil Bernard", "year_born" : 1868 },
{ "name" : "Joszef Rippl-Ronai", "year_born" : 1861 },
{ "name" : "Alfred Maurer", "year_born" : 1868 },
{ "name" : "Edvard Munch", "year_born" : 1863 }
]
}

You can use the $facet stage to perform multiple $bucket aggregations in a single stage.

From the mongo shell, create a sample collection named artwork with the following documents:

db.artwork.insertMany([
{ "_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 two $bucket stages within a $facet stage to create two groupings, one by price and the other by year:

db.artwork.aggregate( [
{
$facet: { // Top-level $facet stage
"price": [ // Output field 1
{
$bucket: {
groupBy: "$price", // Field to group by
boundaries: [ 0, 200, 400 ], // Boundaries for the buckets
default: "Other", // Bucket id for documents which do not fall into a bucket
output: { // Output for each bucket
"count": { $sum: 1 },
"artwork" : { $push: { "title": "$title", "price": "$price" } },
"averagePrice": { $avg: "$price" }
}
}
}
],
"year": [ // Output field 2
{
$bucket: {
groupBy: "$year", // Field to group by
boundaries: [ 1890, 1910, 1920, 1940 ], // Boundaries for the buckets
default: "Unknown", // Bucket id for documents which do not fall into a bucket
output: { // Output for each bucket
"count": { $sum: 1 },
"artwork": { $push: { "title": "$title", "year": "$year" } }
}
}
}
]
}
}
] )
First Facet

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 $bucket stage includes the output document to determine the fields to return:

Field
Description
_id
Inclusive lower bound of the bucket.
count
Count of documents in the bucket.
artwork
Array of documents containing information on each artwork in the bucket.
averagePrice
Employs the $avg operator to display the average price of all artwork in the bucket.
Second Facet

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 $bucket stage includes the output document to determine the fields to return:

Field
Description
count
Count of documents in the bucket.
artwork
Array of documents containing information on each artwork in the bucket.
Output

The operation returns the following document:

{
"price" : [ // Output of first facet
{
"_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") }
],
"averagePrice" : NumberDecimal("140.4375")
},
{
"_id" : 200,
"count" : 2,
"artwork" : [
{ "title" : "Melancholy III", "price" : NumberDecimal("280.00") },
{ "title" : "Composition VII", "price" : NumberDecimal("385.00") }
],
"averagePrice" : NumberDecimal("332.50")
},
{
// Includes documents without prices and prices greater than 400
"_id" : "Other",
"count" : 2,
"artwork" : [
{ "title" : "The Persistence of Memory", "price" : NumberDecimal("483.00") },
{ "title" : "The Scream" }
],
"averagePrice" : NumberDecimal("483.00")
}
],
"year" : [ // Output of second facet
{
"_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 documents without a year
"_id" : "Unknown",
"count" : 1,
"artwork" : [
{ "title" : "The Great Wave off Kanagawa" }
]
}
]
}
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