$concatArrays (expression operator)

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"] }
      ]
   }
]

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.

{ $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" ] } } }
] )

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" ] }
]