$concatArrays (aggregation)

Definition

$concatArrays

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 Expression Operators.

If any argument resolves to a value of null or refers to a field that is missing, $concatArrays returns null.

Behavior

{ $concatArrays: [
   [ "hello", " "], [ "world" ]
] }

[ "hello", " ", "world" ]

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

[ "hello", " ", [ "world" ], "again" ]

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" ] } } }
] )

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