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$concatArrays (aggregation)

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Definition

$concatArrays

New in version 3.2.

Concatenates arrays to return the concatenated array.

$concatArrays has the following syntax:

{ $concatArrays: [ <array1>, <array2>, ... ] }

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.

Behavior

Example Results
{ $concatArrays: [ [ "hello", " "], [ "world" ] ] } [ "hello", " ", "world" ]
{ $concatArrays: [ [ "hello", " "], [ [ "world" ], "again"] ] } [ "hello", " ", [ "world" ], "again" ]

Example

A collection named warehouses contains the following documents:

{ "_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" ] } } }
])
{ "_id" : 1, "items" : [ "chocolate", "butter", "apples" ] }
{ "_id" : 2, "items" : null }
{ "_id" : 3, "items" : [ "pears", "pecans", "cherries" ] }
{ "_id" : 4, "items" : [ "ice cream" ] }

See also

$push