$push (aggregation)
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Definition
$push
Returns an array of all values that result from applying an expression to each document in a group of documents that share the same group by key.
$push
is only available in the$group
stage.$push
has the following syntax:{ $push: <expression> } For more information on expressions, see Expressions.
Behavior
When using $push
in a $group
stage, the order of
the documents in the output array is determined by the order of the
documents coming into pipeline. To guarantee a defined order, the
$group
pipeline stage should follow a $sort
stage.
Example
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 } ] }