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