$concatArrays
Definition
Returns a single array that concatenates two or more arrays.
$concatArrays
can be used as an aggregation accumulator or an
array operator.
Aggregation Accumulator
$concatArrays
is available as an accumulator in these stages:
Syntax
When used as an aggregation accumulator, $concatArrays
has the
following syntax:
{ $concatArrays: "<array field>" }
Behavior
Arguments that are null
or resolve to null
are included in the
results. Arguments that refer to a field that is missing are not
included in the results.
Example
Create a collection named sales
with the following documents:
db.sales.insertMany( [ { _id: 1, items: [ "laptop", "tablet" ], location: "NYC" }, { _id: 2, items: [ "phone", "tablet" ], location: "NYC" }, { _id: 3, location: "NYC" }, { _id: 4, items: [ "desktop", { "accessories": [ "mouse", "keyboard"] } ], location: "NYC" } ] )
This example shows how you can use $concatArrays
as an
accumulator. This example combines the elements of all items
arrays
when grouping on the location
field:
db.sales.aggregate( [ { $group: { _id: "$location", array: { "$concatArrays": "$items" } } } ] )
The operation returns the following result:
[ { "_id": "NYC", "array": [ "laptop", "tablet", "phone", "tablet", "desktop", { "accessories": [ "mouse", "keyboard"] } ] } ]
Array Operator
Syntax
When used as an array operator, $concatArrays
has the following
syntax:
{ $concatArrays: [ <array1>, <array2>, ... ] }
Behavior
The <array>
expressions can be any valid expression as long as they resolve to an array. For
more information on expressions, see Expressions.
If any argument resolves to a value of null
or refers to a field
that is missing, $concatArrays
returns null
.
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Example
Create a collection named warehouses
with the following documents:
db.warehouses.insertMany( [ { _id : 1, instock: [ "chocolate" ], ordered: [ "butter", "apples" ] }, { _id : 2, instock: [ "apples", "pudding", "pie" ] }, { _id : 3, instock: [ "pears", "pecans" ], ordered: [ "cherries" ] }, { _id : 4, instock: [ "ice cream" ], ordered: [ ] } ] )
The following example concatenates the instock
and the ordered
arrays:
db.warehouses.aggregate( [ { $project: { items: { $concatArrays: [ "$instock", "$ordered" ] } } } ] )
The operation returns the following results:
[ { _id : 1, items : [ "chocolate", "butter", "apples" ] }, { _id : 2, items : null }, { _id : 3, items : [ "pears", "pecans", "cherries" ] }, { _id : 4, items : [ "ice cream" ] } ]
Limitations
$concatArrays
only supports arrays and expressions that resolve to
an array.