$push (aggregation)
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Definition
$push
returns an array of all values that result from
applying an expression to documents.
$push
is available in these stages:
$setWindowFields
(Available starting in MongoDB 5.0)
Syntax
$push
syntax:
{ $push: <expression> }
For more information on expressions, see Expression Operators.
Behavior
When using $push
in a $group
stage, the order of
the documents coming into the pipeline determines the order of the
documents in the output array. To guarantee a defined order, the
$group
pipeline stage should follow a $sort
stage.
Examples
Use in $group
Stage
Consider a sales
collection with the following documents:
{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") } { "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") } { "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") } { "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") } { "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:05:00Z") } { "_id" : 6, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-15T12:05:10Z") } { "_id" : 7, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T14:12:12Z") }
Grouping the documents by the day and the year of the date
field,
the following operation uses the $push
accumulator to
compute the list of items and quantities sold for each group:
db.sales.aggregate( [ { $sort: { date: 1, item: 1 } }, { $group: { _id: { day: { $dayOfYear: "$date"}, year: { $year: "$date" } }, itemsSold: { $push: { item: "$item", quantity: "$quantity" } } } } ] )
The operation returns the following results:
{ "_id" : { "day" : 46, "year" : 2014 }, "itemsSold" : [ { "item" : "abc", "quantity" : 10 }, { "item" : "xyz", "quantity" : 10 }, { "item" : "xyz", "quantity" : 5 }, { "item" : "xyz", "quantity" : 10 } ] } { "_id" : { "day" : 34, "year" : 2014 }, "itemsSold" : [ { "item" : "jkl", "quantity" : 1 }, { "item" : "xyz", "quantity" : 5 } ] } { "_id" : { "day" : 1, "year" : 2014 }, "itemsSold" : [ { "item" : "abc", "quantity" : 2 } ] }
Use in $setWindowFields
Stage
New in version 5.0.
Create a cakeSales
collection that contains cake sales in the states
of California (CA
) and Washington (WA
):
db.cakeSales.insertMany( [ { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"), state: "CA", price: 13, quantity: 120 }, { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"), state: "WA", price: 14, quantity: 140 }, { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"), state: "CA", price: 12, quantity: 145 }, { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"), state: "WA", price: 13, quantity: 104 }, { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"), state: "CA", price: 41, quantity: 162 }, { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"), state: "WA", price: 43, quantity: 134 } ] )
This example uses $push
in the $setWindowFields
stage to output an array of cake sales quantity
values for each
state
:
db.cakeSales.aggregate( [ { $setWindowFields: { partitionBy: "$state", sortBy: { orderDate: 1 }, output: { quantitiesForState: { $push: "$quantity", window: { documents: [ "unbounded", "current" ] } } } } } ] )
In the example:
partitionBy: "$state"
partitions the documents in the collection bystate
. There are partitions forCA
andWA
.sortBy: { orderDate: 1 }
sorts the documents in each partition byorderDate
in ascending order (1
), so the earliestorderDate
is first.
output
sets the array ofquantity
values using$push
for the documents in a documents window.The window contains documents between an
unbounded
lower limit and thecurrent
document in the output. This means$push
appends thequantity
values to thequantitiesForState
array for the documents between the beginning of the partition and the current document.
In this output, the array of quantity
values for CA
and WA
is shown in the quantitiesForState
array:
{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"), "state" : "CA", "price" : 41, "quantity" : 162, "quantitiesForState" : [ 162 ] } { "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"), "state" : "CA", "price" : 13, "quantity" : 120, "quantitiesForState" : [ 162, 120 ] } { "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"), "state" : "CA", "price" : 12, "quantity" : 145, "quantitiesForState" : [ 162, 120, 145 ] } { "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"), "state" : "WA", "price" : 43, "quantity" : 134, "quantitiesForState" : [ 134 ] } { "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"), "state" : "WA", "price" : 13, "quantity" : 104, "quantitiesForState" : [ 134, 104 ] } { "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"), "state" : "WA", "price" : 14, "quantity" : 140, "quantitiesForState" : [ 134, 104, 140 ] }