$concatArrays (aggregation)
Definition
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. Formore information on expressions, see Expressions.
If any argument resolves to a value of null
or refers to a fieldthat is missing, $concatArrays
returns null
.
Behavior
Example | Results |
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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